there are soooooo many factors that determine how long an engine lasts.
1)How the engine was broken in
2)Not running it too hot
3)Not running it too lean
4)Keeping your air/fuel clean (air/fuel filters)
5)Storeing the engine with aro
I'm sure that I haven't listed them all, but these are the basics on how long an engine lasts.
I have a Hyper .21 4.5 gallons through it, also a XTM 24.7 with 1.5 gallons. Both are running strong with good compression.
My father in law has a XTM 24.7 pro with 2 gals still strong, he just had his stock Monster gt engine (thunder tiger?) .21 pinched it had 6 gallons through it, would have gone longer but bearing malfunction messed it up.
It all depends how you take care of it.