I normally wear my stuff until it dies and then I give it bandaids until its REALLY dead.
Boards typically will last you between 150-200 days on the hill if you take care of it. (wax, gummy it, repairs etc). For your average rider, the board should last you nearly your whole snowboarding career.
Boots last me one season
Bindings until they break. If the bindings are made well, the first thing to go are the ladders, its an easy fix to replace those and the ratchets too. My 390's are dead to where I have cracks in the baseplate, the ankle strap is worn down to the plastic so I replaced those with some Burton Mission straps I found in the locker room, replaced all the ladders and nearly all the rachets springs have broken. They still work but I'm really pushing for it to last another season.
For outter wear, I wear it until there are holes and tears in it... and then I start using the duct tape.
Its pretty ghetto but I ride a lot.
IMO most people just change their gear because of fashion and new advances in the industry.