some of the "better" battery distributors give some guideline figures - they refer to "DOD" (depth of discharge), and number of cycles - typical figures would be..........

el cheapo leisure battery - 100 cycles to 50% DOD - which means run them down to half capacity, they'll be knackered after 100 times................. the figures are fairly "pro rata", so if you only used 10% DOD, they'd last 5 times as long...........500 cycles

AGM battery - (advanced glass mat) - 300 cycles to 70% DOD (2,100 cycles to 10% DOD)

Top of the range Gel Battery - 400 cycles to 80% DOD

-which means that if you only discharged 10% per day you should get around 7-8 years out of them~!

(these are similar to "golf cart batteries" which are regularly deeply discharged, with this sort of use, they'll do 1-2 seasons.......)

I think the above figures bring into focus the necessity for careful design of battery systems, and choice of batteries (and why it makes a lot of economic sense to only discharge small amounts relative to capacity...)

The 10% "rule of thumb" is fairly accurate - design to never take more than 10% of the battery's capacity per day, never ask a battery to discharge at more than 10% of it's amperage capacity (110 amp battery - maximum current draw 11 amps), and never charge at more than 10% of it's amperage capacity (110 amp/hr battery - 11 amps)