Calculate the charging time of a battery with a solar cell while the battery is being used.
In principle, your ideal case math is correct, however in practice there are a few things to consider.


First off, don't expect the solar cell to deliver its nominal current
at all time, but depending on the conditions it can deliver anything
from 0 to 100% of that (don't exceed 100%).
Secondly, lithium-based batteries are indeed nominally 3.7 V, but their actual voltage ranges between 3 V and 4.2/4.5 V depending on whether you want to get more juice (wider voltage range) or preserve them (narrower range).
When charging, the voltage is typically higher than when discharging for a given charge level, so assume something like 4 V during the whole charge. Add a drop for the charging circuit, which may be as low as a (schottky) diode drop. I think you can get in the ballpark using the current delivered by the panel as the current entering the device.