We can use conservative parameters by studying the ranges of parameters in datasheets of different battery cells in the same size class. I'm using 0.7C charge myself, which is the lowest rate I've seen mentioned in datasheets, and also I've never seen bme charge faster than that. 4.2V, because only only a handful of cells tolerate anything higher, and those are mostly used in laptops anyway afaik.

The open question is thermal, when it's too hot to charge and too cold to charge.