If you run those batteries down to 50 Percent it's not good for 'em. 75 percent is about the minimum for MARINE/deep cycle beyond that they age very fast.
Now I would not replace them till they need it but when they do consider GC-2 Golf car batteries or GC-12's these are True DEEP CYCLE and do not mind 50% at all that's the design limit though.
To recharge
You have about 200 amp hours (Bit less) so at 40 amps to replace 100 AH would take about 3 hours .. But that will only get you to about 90%. the next 3 hours should top you off.. So I'd go 6 hours Might get there in 5 but I'd say six.