All depends where you ride and how much walking you end up doing.
My pair had lasted 2 winters, with only a couple of tiny holes on the bottom from where the grips on my spds had started to wear them. But, wearing them in a wet and muddy Peak District earlier this year has pretty much mullered them. They are still usuable but very badly worn.
Places like Thetford, they work well but rocky places where there might be a fair bit of walking thrown in they don't last long.
With regards to build quality, I had a zip fail on one of mine. They were getting on for 8 months old and were replaced without any quibbles so I wouldn't worry about that too much.