Some properties will pro-rate to by-the-hour and some will have a minimum half day charge, or other, but since you haven't named the property no one can accurately give you the answer you're looking for but you have an easy ability to get the correct info - just call the property and ask the event manager.

Tell them what you want (limited time) and you might be pleasantly surprised or due to timing, i.e. Very early or very late, can negotiate as pricing is not set by Marriott but by the individual property.