If it were current, 3 to 4 hours for a head gasket change would be about right.
The big issue is the liner seals. On anything this old, I would insist on doing them as a matter of course. That's a LOT more work, as everything has to be meticulously clean, dry fitted, and tested before final assembly. It's an oil pan off job to make sure you get all the debris out as well. So you could reasonably double the time to 6 to 8 hours.