You always need to measure and shim the ring gear and pinion properly. Or at least you should. Especially if you plan to drive it for a long time. That process alone will take an experienced mechanic over an hour with all the gauging, measuring, test fitting and marking. And it is much more difficult inside the vehicle than with the rear end out on sawhorses or something.
Bottom line, even if you could slap one in under an hour, it is a great recipe for destroyed or quickly worn out rear end.
Edit: I realize know you must mean like an air locker or open diff conversion kit, something you wouldn't be replacing the chunk for. That could potentially be done in a couple hours if nothing goes wrong.