The charger that comes with the gun is crap but keep it until you upgrade to another charger like what scarecrow sells.
There are a few battery charge time calculators eg:
http://www.csgnetwork.com/batterychgcalc.html
"To figure out how long to charge a battery for:
(battery capacity (battery's mAh rating)/charger output (mA - usually written on the charger)) x 1.4(for NiCad batteries, 1.5 for NiMh batteries) = time (in hours).
In this case it's 1500 / 450 x 1.4 = 4.666hours or 4 hours and 40 mins to charge."
Just read the info from the charger and bettery and plug in the numbers. For the battery first charge I let it charge for a few hours longer.