The length of time, or amount of power contained in the charged battery would be designated by the mAh rating (milliamp hours). However if the voltage doesn't drop on the IMR batteries, a 2000mAh IMR battery won't last as long as a Li-Ion battery with the same rating. However the IMR battery in this example would have
better performance because the voltage didn't drop.
I can try to explain by example. It's all explained by
Ohm's Law.