I give them a certain amount of time, but if not back within that time, then they are marked absent. Sorry, but I don't play that game of students asking to be excused to go to the restroom after I've called roll, and then never return. The door is always open, but getting marked as "present" is my prerogative. They all know that they have four days of not attending my class during a semester. If they want to play that game, then that's one day they are not present.