The moon is the fastest moving planet (technically a luminary) in astrology and astronomy. Most of us are familiar enough with the lunar phases at least. The lunar phases happen because of the sun and moon's position in relation to each other from the earth's perspective. So just to summarize, when the sun and moon are conjunct, it is a new moon. When the sun and moon are squaring each other, it's a first or last quarter moon, depending on whether it's a dexter (right) or sinister (left) square. When the sun and moon are opposing each other, the moon is full.
The lunar phases essentially signify the building up and releasing energy of the moon as the month passes by. In terms of energy, the moon is considered active when it is waxing or full, and less active (or inactive) when it is waning or new. This is why when the moon is new, you'll see people describe it as a time to reset, while during the full moon you'll see people describe it as peak energy. If you follow only those guidelines, that would mean the moon is active for two entire weeks, then inactive for two entire weeks, but of course there's more that goes into it! We don't spend 2 entire weeks doing nothing, even though 2 weeks vacation out of every month would be amazing.
The moon's aspects to the sun are most obvious because of the lunar phases, but the moon forms aspects to all of the other planets too! It's a similar concept. When the moon is applying towards (moving towards) the exact degree of an aspect with a planet, it's active. Then it's less active when it's separating (moving away) from the exact degree of an aspect with a planet.
To explain briefly before I get into specifics, When the moon is not forming any aspects to planets for the rest of the time it's transiting through a sign, it is void of course. When the moon is void, it feels like nothing is happening or you just don't feel like doing anything. It may feel like you're tired or can't focus on big, important tasks. It's recommended to avoid doing or starting anything important while the moon is void. Things started or pursued while the moon is void usually end up going nowhere, or not living up to your hopes and/or expectations. Things usually aren't done as effectively during this time.
So how long is the moon usually void of course? It varies! Most are relatively short. It could be a couple hours, a few minutes, sometimes even just a few seconds. Sometimes they go on for quite a while! Every once in a while they'll last an entire day or even more.
Now, let's look at when the moon considered to be void of course. To summarize, and as I said before, it's when the moon won't apply towards any aspects to other planets before it enters the next sign, but there are different ways of determining if the moon is actually void. From what I've seen, most online calculators will include Uranus, Neptune, and Pluto when determining when the moon is void. Some only use the traditional planets (Sun, Mercury, Venus, Mars, Jupiter, and Saturn) when it comes to this. Either way, I don't think I've ever seen anyone use asteroids or other bodies to determine if the moon is void of course or not. Calculated points, such as the ascendant, descendant, midheaven and medium coeli, of course don't count either since they're not physical bodies.
Most people only consider applying (moving TOWARDS) aspects and not separating (moving away from the exact degree) aspects. Every once in a while I'll see a calculator or person that still gives the moon a separating orb when looking at when the moon would be void of course, but remember separating aspects usually mean an event has already happened, less activity, or inactivity, so most won't use separating aspects at all.
For example, if the moon is in Cancer and the last aspect the moon will form is a square to Mars at 23° Libra, once the moon leaves 23° Cancer it would be void until it enters Leo. Some might give it a couple of degrees, so they won't consider it to be void until after 26° more or less, but I don't agree with that and I don't think many do.
Another thing to consider with orbs is whether you consider in-sign aspects or out-of-sign aspects. Normally, the exact degree of the aspect the moon is applying towards has to be in-sign. So that would mean squares are only considered if it's within the same modality, trines are only considered if they're within the same element, and so on.
As another example for that, if the moon has just begun separating from its last aspect at 28° Cancer, and the next aspect it forms is a trine to Jupiter, which is at 2° Sagittarius, most would consider the moon to be void of course although it's applying towards that aspect and the orb is only 4°, because Cancer and Sagittarius don't form a trine.
There are some people that don't use the moon's ingress (entering the next sign) to mark the cut off point. So some will only consider the moon to be void of course if the moon won't form any applying aspects for a specific amount of degrees ahead of it, even if it ingresses before then.
Say a person won't consider the moon to be void if there's an applying aspect within 15°. If the moon doesn't form any in-sign applying aspects after 25° Cancer, but forms a square at 7° Leo to a planet at 7° Scorpio, they wouldn't consider the moon to be void in Cancer that time since it's within the 15° orb the allow.
The last major thing I'll touch on is that only major aspects are used when determining if the moon is void of course or not! That means only conjunctions, sextiles, squares, trines, and oppositions.
I've seen a couple throw quincunxes (inconjunctions) in there too because some people kind of consider it on the same level of importance as a major aspect, but most don't include inconjunctions or any other minor aspects (semisextiles, semisquares, quintiles, etc) at all. Whether the quincunx is considered a "major" aspect or not is a whole other conversation.
One final note, the book "On the Heavenly Spheres" says that any planet can be void of course using the same rules, and some can be "feral", which means they form no applying or separating aspects to any other planets, and that has its own interpretation entirely. I've seen modern astrologers call this simply an "unaspected planet". Traditional astrology considers the moon to be in a weakened condition if it is void of course in your natal chart. It could have similar effects as a transiting void moon.
Obviously there are a lot of different ways to determine whether the moon is void of course or not, which explains why there are so many different people and calculators that will show different times and lengths for when the moon is void. I'd recommend tracking the moon's cycle for a while and seeing which guidelines for determining void of course make most sense to you. Everyone's different! Personally, I prefer to go by the traditional planets only, applying aspects only, and I don't consider out of sign aspects, but you can decide for yourself!