I was jawin' with an Echo dealer once; his position was that most of Echo's saws are designed to last at least 400 hrs of "average use", which seems to fit right in with what TC165 is saying...3 tanks of gas is probably about 1 1/4 hrs of actual running, times 50 weeks/year, times 5 days/week, times 2 years, subtract an occasional day when there's no light work for the little saw, and you have a little more than 500 hrs of running.
I'd say that was pretty good performance for a machine that costs less than $300.
I'm certain that most comparable models from other mfgrs perform as well.
Let's compare that to, say, a trimmer pushmower in use at a lawn service. That mower will also cost $300, (or more) but certainly will not pass the 500 hr mark without major engine service. Unless maybe you've got an older model with, say, that big 6 hp Kawasaki 2 stroke.