How many hours does it take to change the rear valve cover gasket? How costly?

the fel pros are about 24.00 remove cover, clean channel, stick the gasket with sure tac or anything thin,set easy straight on easy I say ,many miss the mark on setting cover without buggering the gasket, that's 20 minutes .after whatever it takes to expose the cover. I have a love hate relationship with the Seville's and never really did a valve cover job as of yet