MOST areas have a 14 day length of stay limit but in the right location you might stay longer. As long as you have a location that either does not have the limited length of stay or doesn't enforce it, you can stay as long as you like.
OK from reading other posts, you may have been asking how long you can camp without replenishing or emptying tanks. For me with just two people it just about the same as above 14 days. I need to carry an extra 30 gallons of water in plastic jugs (5 of them) and of course would need to recharge batteries every few days but my holding tanks will handle the use as long as we start out KNOWING we'll be there for that long.
Good luck / Skip