Twice a year from the day I buy them. It saved my bacon on my last trailer. When it was about 7 years old I noticed the Dicor on a roof seam peeling up a little. Didn't think it was enough to let water in. Just for grins I sprayed water on it and looked inside the upper storage bins and sure enough there was a little water. I took it apart and found some damp wood. Nothing rotted. Dried it out, put in new screws and used Eternabond tape. Problem solved.
Realistically you won't have to worry about it until it is well out of warranty. However, you'll get to know what it looks like up there and when it isn't right, you'll know.