LOL well for one it all depends on what engine you run. THe cheaper the engine generaly means the less life it will have, not always though. If you always monitor the temps, keep it clean, and rid the engine of excess gas each time you put it away it will last sometime. Now for instance I used to run the os.12' and 15's and usualy got anywhere from 2.5 -3 gallons out of them, now on some of the .21's I have ran Ive got up to 6 gallons on a couple hyper .21 4 ports and 5 gallons out of a couple 8 ports, and have also got 6 gallons out of one of my XTM.247's, my current .247 is rounding 1.5 gallons which is just about its final brakein and is starting to show some serious signs of power, it just keeps getting stronger and stronger. Now Ive heard some of the higher dollar engines like the RB ws7 getting something like 15 gallons+.
What ever your case is, just make it a point to put a new conecting rod in every 2 gallons or so because it will stretch and eventualy fail(snap)