I’m married. Typically women know at least 48 hours in advance when they will be going to the mikvah. In small communities, mikvahs are always by appointment only.
I live in a community in NYC with hundreds of frum families. The main mikvah is in a nearby area. You wouldn’t want to walk there at night, so we have a neighborhood mikvah for use on Friday night only. Appointments are supposed to be made by Wednesday night. I’m sure that if for some reason a woman can’t make an appointment that far in advance, she will be accommodated.