How often should I charge the transmitter?

Every 10-14 days - this only applies to the transmitter supplied with the Libre sensors.

You can check the battery status of your transmitter via the app, but you shouldn’t need to charge it more than every 10-14 days. We recommend that the first charge is between 4 and 8 hrs. The transmitter’s battery gets conditioned each time you charge it. So it may start off lasting 10 days and eventually get to 14 days or more.