How long should caulking hold up before redoing it?

Would be really nice if Dicor would offer their lap sealants in smaller squeeze tubes for touch ups. I don't always need an open caulking tube with the caulk gun. Is there any caulk at Home Depot that would work on our RV roofs? We had new windows installed in the house almost 15 years ago and the caulk around the windows is still in great shape.