I'd leave it at least 6 hours; it's also possible (depending how "new" it is; how long it sat in a warehouse) that it's got the infamous poweron problem, so the first thing to do after you get it charged is to flash to the latest OS...
How long did you let it sit plugged in before deciding it wasn't charging? If the battery is very low I've seen where the unit needs to be plugged in for quite a while before it will even show that it is charging.
So I would swap the batteries to go back to the original battery and let it sit overnight. I suspect you'll have a good charge in the morning.
Now, it'll only stay on if I turn it on with it plugged in, but I cannot turn it on unless it's plugged in first. I will let the old battery charge overnight.