Depends on the time of year and what I'm starting with.
In spring, summer and fall, and starting with a smack pack or vial that is reasonably current, I make the starter (using a stir plate) about 18 hours before pitching.
In winter (with colder temperatures), I do it 24 - 40 hours before pitching.
If I'm starting with a vial or smack pack of old yeast (again using a stir plate), I add an extra 24 hours.
20 - 30 years ago, when I didn't have a stir plate and used to culture yeast from a single bottle with several step ups, it would take me up to 3 weeks to build up the starter.
-a.