Whoever quoted you that labor's on crack! Try 2.6 hours. An AC evaportator job's yet another story.

The heater core is easy to replace w/o removing the dash. There's about 4 screws that hold a plastic housing to the underside of the heater box and once removed (after first removing the firewall connector and pipes), the unit comes out. I've done it several times and I also work at a dealer. Our newest Motor labor time guide says 2.6 hours to replace the heater core but depending on if a "lake" of coolant exists under the floor carpeting/padding and needs to be also removed from your interior, the overall labor could be more. Removing all the coolant under that matting can be far worse of a job to do it right than by merely replacing the heater core as I once had one where the seats needed removal, then all the carpeting and mats had to be replaced as a carpet cleaning place said that they couldn't properly remove the coolant completely. That car was under warranty however so it was done at no expence to the customer.