For new, quality batteries (such as the Panasonics you see recommended here so often), a good rule of thumb is that at the average level of voltage output, a 2600mAh battery will last you 2,600 / 100 = 26 hours of nearly continuous use. If you crank up the voltage using a VV type device, that performance drops.
Performance also drops with time, as "rechargeable battery" does not equal "immortal battery." All rechargeable batteries should be considered consumable and they will eventually need replacing.
Performance also drops with the quality of the batteries themselves. Better batteries last longer per charge and tend to to have a longer lifespan. You can take about 10-15% off of the time between charges for cheaper rechargeables. Even rechargeables "lose" power over time even when not in actual use.
See a top of the line truck battery that has been in a vehicle or equipment for a long time, especially with extensive temperature changes. A relatively new battery refused to start a piddly little 1.5 ton Bobcat after sitting from Christmas through Memorial Day and needed charging.
I use Panasonics in some GPS equipment I use when doing damage surveys. They are, by far, the best brand available right now and they are what I consider "critical assets." Critical assets must work, as designed, when they're needed to.