You can discharge 100% (10.5 volts) and accept shorter life.  Life needed may depend on use.  Say you are part time and and make 10 trips a year and do an 80% discharge 3 times each trip.  That is 30 cycles per year and they may still last 8 to 10 years. 240 to 300 cycles.  Trick is getting them back 100% tip top charged and many converters cannot do this very well.
Otherwise close to 50% is a more practical goal.  But I am not turning my furnace off if I go a bit lower before I can get some charging going.
Full time cycling every day best to stay in the top 40% with periodic extended charges to get back to 100%.