how long does it take for the mouse to charge and how do I know it is fully charged?

The mouse has no visible indicator light or laser, so nothing should be on. The Magic Mouse 2 takes about two hours to fully charge.

To check your device's charge level, click the Control Center icon in the menu bar, then click Bluetooth. Or click the Bluetooth menu in the menu bar. See Recharge the built-in battery in your Apple keyboard, mouse, or trackpad - Apple Support.

Couple of hours for ours.

Is your mouse an Apple product? If not Apple, and Magic mouses have no lights, you'll need to ask the maker or consult the literature that came with it.

But the simplest solution is to plug it in and let it charge overnight as often as needed..

I often leave the Bluetooth control panel open, then connect the Magic Mouse Lightning so I see how much it has charged.

The charging time depends on the charger -- via Mac mini 2018 USB slower and via Mac Book Pro charger faster.