1C or less is typically safe for all LiPos. Some can be charged at more than 1C, and slower rates are better for overall lifetime. For a 150 mAh battery, 150 mA would be 1C. 100 mA charge rate would be 100 / 150 = 0.67C.
Just to be pedantic, battery capacity is measured in mAh, while charge/discharge rate is measured in mA. So a battery that could deliver 100 mA for 1 hour would have a capacity of 100 mA * 1 h = 100 mAh. The same battery could (theoretically) deliver 200 mA for 1/2 hour, 50 mA for 2 hours, and so forth.
Battery capacity is a bit backwards from how we usually think about "capacity". If you ask me how big a water tank is, I might say "ten gallons". With electricity, we think first about "flow rate", which is amps. Then we multiply by the time over which the battery can deliver that flow rate. So it would be like describing a water tank as having a capacity of 10 gallons-per-minute * 1 minute (which works out to 10 gallons, obviously). The difference is that, with electricity, we have a unit of "flow rate", the ampere, whereas with water, we describe flow rate in volume-per-time (gallons per minute) without having a dedicated unit just for that measurement. So we could say that, with liquids, volume is the primary measurement, from which a measurement of flow rate is derived by dividing by time. And with electricity, flow rate is the primary measurement, from which a measurement of capacity is derived by multiplying by time.
