Separate the measurements
Tank capacity is an amount of water. Output is a rate. Dividing capacity by a constant output provides a theoretical runtime; it does not model every operating mode.
Record your own pattern
Start with a known fill amount, record the selected output and note when the device cycles off. Avoid counting a spill or tank rinse as water used for humidification.
Compare like with like
Use the same units and settings when comparing products. Maximum runtime often reflects a low-output setting, while a quoted maximum output may describe a very different operating condition.