Treating 30% of utilization as a rule to be followed is dumb. It is a natural consequence of using a credit card correctly, not something to do for its own sake.
The right way to use a credit card is to buy the things you were going to buy anyway (assuming there isn't an extra charge for using a credit card), then pay the statement balance in full on or before the statement due date. Preferrably by autopay to avoid forgetting. Having interest-charging credit card debt is essentially never worthwhile, and paying the full balance on/before the due date will prevent you from paying interest. Splitting this into multiple payments, or paying for a recent charge before the current statement period ends, is ok but not necessary.
Someone who does this will naturally seek a credit limit that is a few times as much as their monthly expenses. To avoid running into the limit or having to make manual payments, the limit needs to cover the previous month's expenses, plus most/all of the current month's (the due date for the previous month's statement is a few weeks into the current month), plus some cushion. The person will try to increase their limit until it meets those needs, then might not bother increasing it further.
Credit score is a statistical measure, not a prescriptive one. That is, a company will see that the group of people with low utilization is less likely to default (because it includes people who do things correctly), and the group with high utilization is more likely to default (because it includes people who do stupid things like paying interest on a maxed out card). It does not mean that an individual person in either group is doing something good or bad, nor does it does not mean that having a low utilization is something you should do for its own sake. For example, if your card has a few hundred dollar limit, it's perfectly reasonable to use all of it at once, if you then pay it in full. If you use 30 percent of a $20k credit limit, and treat it as debt that you're making the minimum payment + interest on, then it's terrible regardless of the low utilization.
Credit utilization is also a very minor, very temporary aspect of the credit score. If you're applying for a loan soon (a few months or less) and really want your score 10 or 20 points higher to get a better interest rate, then you can play games with artificially lowering your utilization. Otherwise it's not something you should even need to think about, other than eg the convenience aspect mentioned above. Making payments on time and not taking on pointless debt is all you need for both a good credit score and good credit "health".
The above was in response to "I heard it's best to have low utilization," which is a horrible misunderstanding. To answer the direct question: After a given month ("statement period") is over, a statement is created with that month's charges, plus any debt left over from earlier, plus (only if there is debt left over from earlier) interest. Payments for that statement are due usually a few weeks later. Utilization is reported according to that statement balance, which doesn't include charges or payments made later. Eg, it doesn't include whether you pay it in full or not. Payments are applied to the previous month's statement before the current (not yet over) one. So if you pay for everything from the previous month, then (before the current month is over) make extra payments for things bought this month, then those charges won't show up as utilization.