I seem to only get 4-5 years from a propane grill until the flare ups are near constant.
Thorough cleaning, with grill cleaner, and taking everything apart for the cleaning does help. But after a few years the flare ups happen frequently regardless of me cleaning. It seems the only way to fix the problem at that point is to buy a new grill.
My last couple grills have been in the ~$1,000 price tier from Weber.