covered wagon,
I'm in the 4 year range with 3 type 31 AGM's. They have been trouble free so far and the only maintenance has been to clean the terminals each year.
My wife and I lived on a sailing yacht for 23 years and had extensive experience with various sizes, brands,and prices of batteries. We found on average that after the third year we could expect batteries to lose effectiveness. Sometimes some would go longer, some less.
I would not consider going back to lead/acid, the benefits of the AGM are much less corncern. They are a vast improvement in battery tech. They appear to take a charge faster, hold the charge better, I do not have think about the batteries as much. Until the next improvements are released I'll be using AGM's. The DEKA Brand was the best priced when I was looking.