Stan-
IMHO/Experience:
Quality 12 v deep cycle batteries used in regular service and taken care of (not deeply discharged beyond 40% or so, recharged soon, fluid checked, plates de-sulfated, battery top and posts cleaned, etc.) and used and stored in moderate temperatures, usually last 5-6 years or even more, occasionally.
Routine preventive replacement at 5 years should be safe, if the batteries are watched to see if they recover their full voltage after charging and hold it during storage.
High or low temperature extremes and taxing usage patterns can shorten this, but I don't have enough experience at those conditions to give you specific advice except to watch them as above in the previous paragraph.
You should be OK, I'd guess, at 3 years.......?
Joe. :teeth :thup