Properly maintained batteries could last forever. If you took care to never discharge the battery below 80% capacity prior to a full recharge; they say they will last indefinitely unless they are abused (Discharged then frozen; dropped; bounced; or shorted out).
When discharged deeper than 80% before a recharge; the number of charge/dishcarge cycles possible starts to decrease. Most deep cycle batteries will last about 200 full discharge to dead then charge to full cycles until the amount of charge they are capable of holding drops to a point where they are unusable. Most never abuse our batteries that way and get better life out of them.
So, if they still keep a good charge and you are happy with the capacity; they should last another six years.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW