Moss balls can live forever like all plants. I've had mine for as long as I've had my tanks. Every time that I change my water I take my moss balls out, rinse them, give them a squeeze and drop them back in when I change the water.
They help keep the water clean by feeding on the same nutrients that algae do, and therefore they keep the algae out of the tank because algae can't thrive if the nutrients aren't there. That and they're a plant and you know, provide all the same benefits that any other plants do, without needing to put down a root system. That and I just love the way that they look :)
Male Bettas: Jasper (PK), Gold Dust (PK) Iced Steel (EE HM)
Other: Faith, Frankie, Sonic, Ashley & Anna (Juli cory catfish), Mr. Bristles (albino bristlenose pleco), The Shrimples (dozen ghost shrimp)