my experience
i have 3yr old "surplus" AGM's removed from (internet backup server power) when they were three yrs old..sat on float for 3yrs
they have been in daily use for 3yrs, now 6yrs actual age..3yrs daily use, that about 1100 charge/use cycles
they are still good.. just a few weeks ago i did a series of Load tests on them with a carbon pile load tester, each and every battery tested good
each battery weighs 100# and each one is 139 ampHrs