Long enough to get you home or to the shop for repair no. If it's an open diff or e-locker (non-anti-spin) then you're definitely okay.
As cj7 mentioned, if you are going to do any real distance, regardless of rear end, I'd put the spare on the front and put a good 'normal' tire on the rear to even it out.
I always replace my tires in pairs at a minimum so that they are as close to the same circumference as possible, but in my younger years I was told to use the 5% rule. As long as the circumferences are within 5% you're okay but that was "back in the day" where information was passed down from gearhead to gearhead (shops, friends, parents, etc.). I don't know how accurate that is today with the advancement in vehicles (manufacturing, technology) as I don't have to stretch my dollars as thin as I did in my late teens and early 20's.