I've been running on a two-battery bank for about 8 hours with a shot
alternator. Is this normal?

Batteries are two weeks old, crankin' batteries. Engine is a 5.7litre
'78 Mercruiser. Cabin guage shows this bank at 12.2v, other bank
which hasn't been used at all is at 12.8. No increase in voltage when
engine is running, on alternator tester or two other on board guages
(ammeter and voltmeter) No other house current being used to speak
of.

My other boat is a 15' Bayliner with a 50 horse Force, so obviously I
have no basis for comparison! I'm taking alternator in to the shop on
Monday. How long can an engine like this run with no alternator?

Thanks in advance!
Speedo