I can't seem to find the mAh rating for the MS battery packs, but you can easily get 2 rechargeable AAs with 4800 to 5200 mAh total. Unless they've upgraded the batteries over time, that probably comes close to what they are. Those should last a good while if you want to go that route. I would suggest that really, since buying some rechargeable AAs and a charger would also work for a lot more applications and over time would be cheaper than buying AAs over and over.
EDIT: I just looked at a controller and it's rated at 450mA (which should be the maximum), so 2 2400mAh AAs should last at least 10 hours, and 2 2600mAh AAs would give you at least ~12. Since MS says their rechargeables can last "up to" 25 hours they probably hold more power than that, but I dunno what numbers they're using for that estimate.