I second the osmolator. Primary switch isn't a float (backup switch is a float)...used the same one for nearly 8 years now no overfill problems at all. I did have to replace a couple pumps which only are about $25...but I attribute that to pumping kalkwasser.
About a month. I can tell you my salinity has been alot more stable. My tank evaporates between 1 and 2 gal a day, so a couple days of not topping off could really let it creep up there. Just makes more time to stare in the tank.