One of those $14 50 foot rolls from Walmart will be more than enough. Or you can order an 135 foot roll of 14 gauge ( some say 16 but really doesnt matter for your application ) speaker wire form audio savings.com and have enough for whatever else you will want to use it for in the future.
I would allow between 15 and 20 feet per door depending on where all you have to run the wire. The walmart recommendation above is a decent one because you won't spend that much and will have extra. It's always better to have too much than not enough.