Sounds like you mean what return time to put in when you book the reservation. Just book a 24-hour period. That's always safest. No need to book shorter and then have them decide you are "late" even though it's still within the same 24-hour period.

For multiple days just always use the same return time as your pickup time, however many days later is necessary. Unless you are a whole day early on a 5-day or something (which might bump you back to the daily rate vs weekly) you won't be penalized for returning early.

Indeed, when I do some midweek daily rentals for work, I'm sometimes not sure if need the car for 2 days or 3, or sometimes they won't let me pick up a car unless I specify at least a 2 or 3-day keep. In those cases, I just book the longer period of time. If you return a whole day early, they will only charge for the number of days you kept it. Last rental I couldn't even get a reservation unless I specified 3-day keep. I returned it at only 2 days, and they charged me for only 2 days. I've done this several times and never had a rate change on daily rates.