Run time = mAh/mA
is a reasonable approximation for LiIon cells at around 1C discharge rate and 20 degrees C. At very low discharge rates if the effects of self discharge are ignored then the effective mAh capacity will be higher. Increaases in capacity with decreasing discharge rate is less pronounced that with head acid chemistry.
With LiIon an increase in capacity in the 10% - 20% range is liable to be experiences compared to rated capacity.
Note that the discharge range is 4.2V to 3V - 3.6V or 3.7V is the mean voltage during discharge.
For lead acid chemistry Peukert's law may be used to estimate battery capacity.
It is generally considered that Peukert's law is far less applicable to LiIon chemistry.
This paper claims that Peukert's law is 'differently applicable' to LiIon with alpha close to unity and a polynomial law said to be more appropriate. Observation indicates that LiIon has a much lower change in capacity with load than LA.
This paper discusses applicability to LiFePO4 cells.
Paper re applicability to LiFePO4
This paper argues the law cannot be used even for LA except at constant current and temperature.