Whenever you have an open ended timeframe in which you need people to self select when they want to come in, you can set up Events with TimeSlots ability. There are many instances in which you want end-users to select a "minimum" timeframe to register to.
e.g., if 15min is selected, only 9-9:15 or 12-12:15 or anything more than 15min is allowed, otherwise it would "throw" an alert or error. If you chose 3 hours, individuals who registers to the timeslot will need to select 3 or more hours to register with. Otherwise they will see something like the below: