Putt,
1/2 A's will fly forever on a few ounces of fuel. In fact, many come equipped with a one ounce tank mounted to the back of the engine, and will fly for 8 minutes or so.

I'd highly recommend using Norvel engines in your 1/2 A's if thats the way you go. The Norvel 061's run better and are easier to set up than the Cox engines. (You can get them with the built in spring starter too, if you want.)

Dennis-