Depending on the condition of the pack, I would guess somewhere around 8-10 hours. You may get a bit more with intermittent use rather than continuous use. If it's the stock Minelab pack, you might want to run it down in the machine until the battery alert comes on, and then recharge it for about 14 hours with the stock wall charger. Then do that same cycle about 4 or 5 times to erase any memory that may have built up in the pack. I connect my coil to the box and just turn the machine on with low sensitivity, and just audible threshold, since it's in the house, and run it until the low battery alert comes on. Then go through the charge/discharge routine. I'm still using a stock Minelab NiCad pack that is 12 years old by using this method.
Hope this helps,
HighPlainsHunter