First, my idea of a lawn mower needing replacement is different than that of most here, something I didn't think about when I posted. I don't actually run my mowers until they completely die, I run them until they start burning enough oil to concern my wife, who knows too much about the dangers of motor emissions.
My yard is .4 acres. That's not so bad, but there's probably about 2 floors worth of yard grade front to back. Since my mowers are self-propelled, I imagine I'm taxing the engine pretty hard. Furthermore, my backyard is next to a stormwater runoff creek, so the backyard grass is very thick and is often damp well into the afternoon. I frequently mow it damp and the engine chokes out a lot, which can't be good-- but hey, it keeps the neighbors happy.
I imagine my yard is harder than most on a mower, particularly one that is self-propelled.