To add to the last post, I am a weekend racer that usually runs one day a week (sunday). In the last two seasons I used 4 and 4.5 gallons of fuel. I don't practice much though.
An engine can last a long time (7-10) gallons but when an engine is "worn out" is determined by the user. You will see guys claiming to have an engine for 10 gallons but only a couple I have seen with that kind of time on them ran well and the ones that did were owned by expert racers.
"I have no idea what you're talking about, so here's a bunny with a pancake on it's head."
In mind and body, we're all heading towards old age. It is slower to go by bike.