I consider myself the Crew Car king
...
...and it DOES depend on the FBO. Big and/or busy FBOs will have time limits (i.e 1-hour to grab lunch or run an errand). Smaller FBOs are pretty liberal with what you can do and how long you can stay out. Best thing to do is call ahead and let them know what you want to do; they might "reserve" it for special occasions, otherwise it will be first-come, first-serve of course. It also helps to call and see if a car is available at all (some places refuse to have crew cars because of insurance/liability nowadays, and some places may have their car in the shop when you want to use it). Been to a couple mostly-unattended places that leave the keys in the car 24-7 and post a sign in the FBO that explains to transient pilots that they can just grab the car and go...And some places will let you take it overnite if you bring it back early (as has been said).
Regardless, a crew car/courtesy car is a COURTESY and if you have the time/money, top it off for the next guy when you use it, and get some 100LL to help out the FBO, too (even if you don't really need to top off the plane, it's a good courtesy). If it's a busy place and others are likely to drop in and use the car, you may wanna think twice about keeping the car out for a long time.
AND...don't abuse the car or wreck it, either (your insurance may not cover a "borrowed" car, and some places don't insure their crew cars properly, as I found out the hard day one day when I backed a crew car into a tree by accident).
BTW, the worst crew car ever is located in BFR (Bedford, IN). I've driven that piece of crap a number of times over the past few years, and I wasn't surprised when AOPA did and article on crew cars last year and the BFR car made the list of worst crew cars.
It's a tan Chevette, 50 percent rusted away. Barely starts, barely gets you to town, barely gets you back, sounds like he11, etc. etc.