A question on battery life calculation
With a switching mode converter you would use a power conversion formula. You do need to consider that some of the electrical energy will be used up in the converter and converted to heat. So the formula becomes:

Time = (Input Capacity * Converter Efficiency) / (Output Power)

If for sake of simplicity you could consider that battery voltage stays constant during the time the battery capacity is consumed that becomes:

Time = (24V * 50AH) * (0.78) / (5V * 0.1A)

Of course your efficiency may be different than 78% and the 24V is likely to decrease as the battery becomes depleted.