Most modern batteries do not fail suddenly. You should see some warning signs as your battery is going "soft", like headlight at night dimming at idle/brightening with throttle, or dimming/brightening when the turn signal is on.
You are doing the best for your battery by maintaining the electrolyte level and keeping it topped-off on a maintenance charger. Lead acid batteries hate to be discharged and re-charged. Causes sulfation of the plates, which essentially removes plate surface from exposure to the electrolyte and therefore reduced capacity. Keeping lead acid batteries topped-up at all times reduces deposition of sulfates. Certain chargers (BatteryMinder) actually use a cycle which includes short pulses of "charge reverse" which supposedly restores sulfated plates somewhat. I'm not sure that this really works, but I buy those types of chargers/maintainers anyway, just in case it does.
I don't think that you really need to replace your battery until it starts showing some signs of deterioration.