Hi KendallP,
How long is a device supposed to do "what it is designed to do"? If there is no warranty then the answer is forever. Warranties do not exist to protect the consumer--they exist to protect the manufacturer and limit their liability.
If Mr. Wizard doesn't hit "break even" on energy soon, then the only way to approach that figure would be to buy more panels, assuming he has room to install them. If he does not--then the unisolars may have to be replaced with panels that do more "watts per square foot".
I've got room for two more Unisolars and will buy his old ones *grin*.