Snibble
Roughly 6 feet per door. It is better to have too much rather than not enough. Try to use 16 or better yet 12 gauge wire with the appropriate connectors ( no twist and tape ). Your local audio store will probably have some highly priced speaker wire available but your local hardware store can save you a grip of cash. I have used zip cord (the same wire as a brown or white extension cord) from Home Depot with great results. Dont let some one talk you into $2 per foot speaker wire and use terms like "shielding" and "skin effect" or "RF attenuation". Good luck and triple check EVERYTHING before you power up.
You are gonna have some fun. Check out the link I pasted on
goldstones Ohmmmmmmm post. It has some very helpful advice.