How long did it take you to rebuild your

Depends how bad of condition it's in. If you need to repair or rebuild parts of the case with fiberglass and/ or adhesives, it could take a couple of days. If it's perfect, (but what ever really is??) an afternoon isn't out of the question, but it all depends on what you're starting with and what your doing to it. Heck, it could take most of an afternoon just to get a new heater core to fit right...
Just do it and you'll see how long it takes. It isn't going to change the fact of whether you are going to do it or not.