That aftermarket pack that I think you are talking about is a 1600ma nimh pack. The Sovereign/Excal drains about 50 to 60 ma from memory, depending on if it's sounding off to a target or not. High balling the amp draw to 70ma just to be safe, 1600ma divided by 70ma = 22.8 hours of hunting time.
Really a good healthy pack usually holds more capacity than the label says, like say in the 1800 to 2000ma range. That's one of the advantages to using a charger with a capacity display, so you can see if the pack is in need of excercise. Only is useful information though if the pack is drained dead before charging because it won't show the total capacity the pack is holding if it's already half charged when you start charging.
If the pack holds around 1700ma you can figure around 24 to 25 hours per charge then. Problem is that unless you are not using low self discharge nimhs a week later or so the capacity just sitting on the shelf has already dropped a good bit. I bet you'd have no problem though getting about four hunts in in a week of about 5 hours a piece before the low battery alarm.
I'd probably re-charge every three 5 hour hunts in say a week or two's time or so, so there is never a risk of running into the low attery alarm. If you've got a spare back then just hunt the first one until the low battery alarm. Healthier for the pack IMO to drain it down fairly good here and there to at least low battery alarm, but for me I drain my packs completely dead on a car tail light bulb 2 or 3 times a year about 3 straight times in a row to excercise them and get more capacity/run time. A new pack needs excercise too to greatly increase it's capacity, so I'd at least run to low battery alarm for about 5 different charges and then by then you'll be seeing much longer run time out of the pack.
If you are using the stock wall transformer to charge it, look on it and see what the output MA is. If it's 100ma then that's 16 hours to put 1600ma back into the pack, but if it's completely near dead then I'd shoot for about 20 hours due to the in effeciency of the charging process and also the fact that the pack probably holds a good bit more capacity than 1600ma.
Easy way to figure out how much to re-charge your pack is figure the # of hours you hunted and X that by .70ma to figure the aprox capacity taken out of the pack. Then divide the output of the wall transformer into that to get the aprox # of hours to re-charge it back to full.
Really if it were me I'd buy at least one of the simple plug-n-play chargers they sell for that pack so you've got a clear indication when the pack is charged. I hate the Minelab wall transformers because they don't tell you when the pack is charged and take forever, although I guess the Elite/GT packs lights are said to go out when they are charged because of the little charging circuit inside the pack.
If you use an aftermarket charger don't use one pushing it past about 800ma for the pack, which will take two hours to charge a dead one. That way you know you are being easy on the cells for long life, but it's pointless to charge at the super low rate of the Minelab wall transforms. Well beyond what is needed in a slow charge to not push the pack. Some guys will charge at 1C all the time (1.6 amps for that pack) but that might be pushing the back and give it less life/capacity over time if you do that fast every single charge. Besides, I've read of guys who say the POD gets real hot at high amp rates and it led to the plastic cracking over time. Even at 1/2 C ( 800ma) I'd feel that POD and see if it's getting hot because the POD is holding in the heat. A battery should never be very warm or hot when charging. Luke warm to the touch is OK, but still is a sign the pack is being pushed a bit, because heat destroys batteries over time.
