You can reuse a pair as many times as you like. I'm still using the pair I got wen I saw Up nearly two years ago. They don't expire.
The recycling bin is for people that don't want to carry the glasses home, and helps the cinema out because next time the customer comes in they pay the extra dollar for another pair.
Even if you have a pair of glasses you still have to pay the $3 or so "surcharge" for a 3D movie.