Not sure on the explorer. The etrac I charge overnight. I also got rechargeable batteries and put them in the alkaline battery pack. They are Duracell and those take about 2 hours to charge. They have the green light that lets you know when they are charged. My factory rechargeable battery pack never gives you an indication when its charged but I have heard they are engineered to not overcharge. I hope this helps you some. Maybe someone with an explorer can chime in. Which explorer do you have, the se pro, xs, ?
A lot will depend on the degree of starting charge and the age of the battery. Usually 10 hours for a dead battery in good shape. The aftermarket RNB batteries take a max of 2 hours to charge and last 4-5 times longer than the stock one.