You have a truly Magic Mouse if yours charges in 15 minutes. In my experience, it is hours to charge from zero, which until I put an always-running monitor in the menu bar for the mouse battery level is what you are guaranteed to have since there is no other indicator of mouse battery level.

I used to roll my eyes at the complaints until I actually had one of these, and it is appallingly bad engineering. Especially since the previous design, which was functionally identical just needed a 10 second battery swap.