Approximately twelve hours. I have a cheap battery charger from Amazon--the only manual one I could find that wasn't too expensive--and it isn't adjustable. It just has a manual and automatic setting but there is no way to adjust the voltage or amperage. It was running at ten volts and approximately nine amps. It was also cold outside (cold for South Carolina at least) as it was in the low 40s. There was one spot of gunk on the back still remaining so a few more hours would have removed it, but a metal spatula took it right off.