Capacity in mAh (milliamp-hours) is charge stored. Runtime is capacity divided by average load current, adjusted for cutoff voltage and converter efficiency.
Simple estimate
Hours ≈ mAh / (1000 × Iload)
Example: 2000 mAh cell at 50 mA average → about 40 hours before the pack is depleted, ignoring cutoff and temperature. The battery life calculator applies derating factors you choose.
Peaks versus average
Radios and motors draw pulses. Size the pack for average current but verify the cell can supply peak current and that your charger IC matches cell chemistry.