Is it possible to accurately calculate how long a Li-Ion cell takes to charge?
If you want to be precise, you should map the charging cycle of the battery (e.g., the voltage vs. current during the charging cycle).  Usually a 2-piece linear approximation of current is adequate, but it will depend on the battery capacity and the charge rate to find the knee.  Once that curve is established, the area under the curve represents the A-H delivered to the battery.  If you know the capacity and SoC, you can use that to determine the time to full charge.