Purchased new battery; how long should it last?

This 2008/2009 MacBook has had endless issues since purchase with charging/staying charged. Battery tests have always indicated that it was fine. Borrowed a new charger and that seemed to help though we got to where we only ran it off the charger.

This computer has always had light use; mostly for projecting Powerpoints.

Recently tried to use it for other applications and it would suddenly quit (black screen) while on battery or if I unplugged the charger despite saying it was charged. At one point, it dropped from 60% to 0% in an instant. I tried a battery reset and it didn't help.

I decided it was time to try a new battery so I ordered one via ifixit. I have fully charged it and I am now running it to discharge it so that I can recharge it again, as suggested for "priming" the battery.

The % fell pretty quickly from 98% to 65%. (Yes, I have checked what is running in the system, the brightness, etc.)

I still need to figure out if the issue really is the battery. My question is, about how long should I expect the computer to run under light use with the new battery?