Recommendations vary by product manufactures, but as a general rule, you should re-seal your deck when the stain breaks down from normal ware or from UV rays. Decks and fences should NOT need to be re-stained every year or two. If your deck is peeling after one or two years, the contractor is using a cheap and inferior stain. In most cases, the contractor did not sand your deck as a part of the preparation process and as a result, the deck peeled.