In an ideal environment, the M215 2nd Gen mouse can operate up to 82 feet (25 meters) away.
If you're not getting this distance, try these suggestions:
Replace the battery/batteries or make sure your mouse is fully charged
Move devices that emit radio waves or could cause radio interference away from your work area (Examples: cell phones, radios, wireless routers, microwaves)
To determine if your environment is shortening your operating range, try using your device in a different environment to see if the distance improves. If it does, look for other possible sources of interference you can remove from your work area.