Have the battery load tested at an Autozone or other parts place-- it's free, and the best indicator of when it's time to retire a battery.

Not sure what "too long" is to you, but I've had to jump or charge (depending on the urgency of a trip) our 245 camper/pickup truck when it has only been a week, and other times it starts up after three or four weeks.

The battery is 5 years old but that's new for our fleet. I suspect the usual frayed insulation on the driver's door central locking switch may be intermittently sucking the juice. Just haven't been able to catch it in the act yet.

When using it for a camper, I thought to check the parasites* before retiring for the night due to the inconvenience of getting a jump. The battery itself seems to be holding the charge, warm or cold.

Suggestion to self: Disconnect door switches from central locking. Easy to do right under an '89 245's dash, and non-issue given we have keyless entry on these cars.

*remove the negative terminal on the battery and measure current with everything turned off and the doors closed.
--
Art Benstein near Baltimore

Anybody who doesn't know what soap tastes like never washed a dog.
-Franklin P. Jones

If you can boost the car and run it for a few minutes, turn it off and then start it without problems, the battery is probably okay. Then just give it a full charge.
--
1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

You've probably already answered your own question but try and slow charge the battery for 24 hours, or longer, and see how it recovers. Should charge up to about 12.8v. It will drop a bit but shouldn't go down to less than about 12.6v. If you take a fully charged battery to a battery shop they can do an electrical "drop" test on it where they do a quick and dirty total discharge on it and see what the meter reads. Bottom line, buy a new battery and go from there.