These guys are all correct - too much sun and/or inclement weather will shorten the life of any sealer. Usually, the sealer is still working but the appearance is bad. I have also found that when pets spend a lot of time on a deck that it can affect sealer life.
Its funny how we are selling protection but the customer is buying appearance. Its a miracle we can agree on anything!
Here's what I did: I used to tell my customers that the sealer is designed to last 2 years on horizontals and 4 years on verticals, but that time can vary according to conditions. Then I offered the customer a pro-rated guarantee over the time span. People who called me after 18 months got a 25% discount - but I kept the customer and the customer was satisfied. Besides, we were the most expensive service in our area to begin with.
I hope this helps.