How long do I charge the battery for a "Power Grinder" brand rotary tool?
It's normal for a battery to self-discharge over several months. 
It's normal for a cheap tool (Black &amp; Decker tier) to take 12-18 hours to fully charge.
What damages such tools is being left on the charger for days and days. 


The reason is because their charger is not using "everything we know about batteries". By contrast, a Tesla supercharger can calibrate charge rates to take a Tesla Model 3 from 25% to 75% in 25 minutes at a rate that would take another Tesla from 90% to "fiery explosion".  Most batteries want a bulk charge rate in the middle of their range to about 85% and a low current taper charge near the end of cycle.   

In the case of cheap chargers, rather than shift gears from "bulk slam-charge" to "low taper" to "maintain", the cheap charger simply does the taper charge continuously! 

Which makes the charge comically slow.  But results in de minimus risk to the battery.  

Note that it doesn't transition to a maintenance charge either; that's why you can't leave it on charger indefinitely.