Charging your RepOne Sensor can be a bit of a guessing game with 0.5.X firmware, as there's no battery fuel gauge (battery %) and no charging indicator. These are coming in 0.6.X which will be released in September 2020. In the meantime, here are some best practices.
In our battery life help article we outline how you can estimate how long your battery will last. We suggest cutting that in half or in a third, and charging at that interval. For example, if your use estimates 3 week battery life, charge every one to one and a half weeks. Once 0.6.X firmware is released you can simply charge when your battery is low.
Charging the battery takes approximately 1.5 hours from dead to full. You can get enough charge from a dead unit for one session in about 15 minutes.