When it comes to landing gear (which is constantly the first thing to go on my planes that have them)
there are two methods I found that seem to work well depending on the bird.
the first is you strap in some basswood strips or plans, and the landing gear is actually bonded/bolted to that. It spreads out the load to a much larger area for only a little more weight. this works best on larger planes like the Bushwhacker, the edge 540, etc.
On the smaller planes I take the alternative route, I make the landing gear removable, and springy. I support where it goes into the plane with Popsicle sticks, but the gear itself is basically bonded to some foam-board and pushed into the plane in a slot. if the gear bends or is damaged it's really easy to remove and fix/replace. Alternatively you can mount your landing gear to a plate, and use rubber bands to hold the plate to your plane's underbelly. that way instead of breaking the plane the bands will give it some forgiving spring (or snap and throw off the gear but it should be less injury to the plane overall than fixed gear that TEAR out of the plane.
As for how long they last, that STRONGLY depends on the pilot and use of the plane. I've had planes last two flights and they started falling apart. but the very first mighty mini scout we ever built is STILL flying and that thing has taken loads of abuse. I actually am more likely to retire perfectly good flying planes simply because I keep building too many, But I've had some I really liked that took just one too many bad landings and it ended it's career.
I guess I had not considered recording my number of flights or hours on any given plane before.. I should start to do that, it wouldn't be hard to do!
I think the biggest draw to foamboard planes however is just how cheap and easy it is to repair and replace parts on them, I can smash a plane in a nosedive crash, and by the end of the evening it will be ready to fly again. (although not fully re-painted)
Foam planes will never STAY pretty. I have a few I spent a bit too much time on getting the paint JUUUUUST right on them, and every time I do it's like I placed a curse on the plane to fail. The more time I spend painting them the more scared I am to fly them, and the more likely I am to crash them. I agree with others here, the uglier the plane, the more durable it seems to become. The pretty ones come and go but the crap-tastic planes take abuse all day long and don't seem to want to die no matter what you do to them. That simple scout I mentioned earlier I've tried to fold the wing on it, pushed it into VERY high speed dives and loops on a 2700kv 2205 motor with a 6x4 prop.. I've done crap to that plane thinking for sure it would die... and no, it's still in our hanger, still flies great as the say we made it.. even if it's had a bit of extra hotglue added over time added to keep it flying.