Thank you for your question. The standard answer is that an instant lasts for no time at all. That is to say, the start of an instant and the end of an instance occur at exactly the same time. An instant is indivisible; it has no separate beginning, middle, or end. You might think of time as like a number line, with (for instance) zero as the time when you started reading this sentence and 1 as the time when you arrived at the end of it. Then each number between zero and 1 corresponds to an instant of time. None of those instants is any length of time at all.

Of course, that an instant of time lasts for no time at all might lead you to wonder how a span of time lasting, say, for an hour could possibly consist of a bunch of instants each lasting for no time at all. This is closely related to some of the paradoxes first proposed by the Greek philosopher Zeno thousands of years ago.

Bear in mind as well that between any two instants of time, there is another instant of time -- and, indeed, infinitely many instants of time, since you can repeat this thought infinitely many times. But that should be no more surprising than that between any two numbers, there is another number -- and, indeed, infinitely many numbers. If an instant of time lasted for any length of time (other than zero) and all instants lasted for the same length of time, then you couldn't fit infinitely many instants between any two instants.

So in thinking about how an hour of time could consist of many instants, each lasting for no time at all, you have to bear in mind that an hour consists not just of many instants, but of infinitely many instants. I hope that this answers your question (a little).