The hour is from 00 minutes to 59 minutes (top to the bottom of the hour)
Example It goes from 9:00 to 9:59, it does not go from say 9:30 to 10:30
In my experience if you miss one hour the next hour you will not get a stand reminder.
Example: If you are up an moving between 8 & 9 there will not be a reminder as you are moving
If you sit from 9 to 9:50 without moving you will get the reminder.
If you move it will go away until the next hour 10-11
And if you don't move in that time it will you will get a warning at 10:50
If you don't make that warning you will not get any more until you move as the watch will assume you are sleeping.