Also not sure how many hours, but it requires removing the dash, just for starters. I rate it a 7 on the PITA scale.
Before you go ape, check to see if it's just a hose leaking before you go further. Odds are that it IS a heater core, but you might get lucky.
The carpets on the passenger side will be wet if its leaking inside the car. You might also see steam coming out of the vents when you crank up the fan.
Get some estimates from several garages. Work rates vary, but they all reference the standard book time. It just depends on how many dollars they want for each hour (plus parts and supplies, of course).
"You can never be free until you let yourself go."