Hello and welcome to ER beem0!
You can monitor battery pack voltage to know when you've gotten down to 50% dod (depth of discharge). Right now, I've just been using a voltmeter across all the batteries. I plan on using an individual LED battery monitor (3 bar graphs basically) very similar to
this. A guy from the local electric car club has designed a similar unit and will be making a batch of them.
I've been going over my battery choices and I think I am going to settle on three group 31 12V batteries. These won't give me the run time I really want, but the more I think about it, the less I want to lug around six 6v batteries. The mower is already heavy enough with Ben's three gel cells on it. I don't even want to think about three more batteries of that size. Plus, I have to mount them all too. The group 31 will be rated a tiny bit over 100Ah and give me about a half hour of run time to 50% dod. This is acceptable. I also looked at the numbers, and assuming I used the mower once a week and ran the batteries down to 100% dod, I would still get roughly 11 years out of them. If I went down to 50% dod, I would get 22 y ears. I'd have to double check that though. So, going to 100% dod, and my 1 hour of run time is possible for a hit to the life of the batteries.