what is average hours of labor to repair heater problem?

Removing the dash may take 15 minutes if you know the procedure , I don't think it's an indication of job difficulty . The part may have to come from the dealer depending on what the problem is and that may be $$ , Cleaning the contacts on the selector switch may solve the problem who knows .

On a twenty year old Chevy, the problem might have been just engine coolant being too low to reach the heater core. If this work was done at a dealer, it's possible, or probable, or a certainty, that you were taken and the dealer's service people took you for a sucker, but then that's how they look at most customers. As my buddy, beatupchevy, said, who knows: it might have been a defective mode door.

I just assume the coolant level is topped off and both hoses going into the firewall are warm/hot and there are no air pockets . .

I just assumed this guy was like everyone else who asks questions here and he needed help just getting the hood open. And if asked if he checked the oil level, he'd reply: the WHAT?