Magic mouse with usb-c

Lithium batteries lose their capacity over time, although three years may be a bit too soon. I would be more concerned with, whether or not, it can be fully charged than the time it takes to charge at this point.

However, if you are pining for the new USB-C Magic Mouse, the only key is that its full functionality requires macOS Sequoia 15.1 or newer ... which it appears, from your tagline, that your iMac is already there.

The Apple Magic Mouse can show 100% battery in the macOS System Settings under Bluetooth or Mouse settings, but this percentage is an estimate rather than an absolute indicator of full charge. Apple designed the Magic Mouse with a lithium-ion battery, and these batteries often report their charge in ranges, which means 100% doesn't always equate to the absolute maximum capacity of the battery over time.

In other words, it could read 100%, but its actual maximum capacity had declined to half or less of its original capacity. For example, if it has declined to 20% of the original capacity, then it could still be charged to 100% of that 20% capacity. The end result is shorter battery useful time before it has to be recharged.

Typically a full charge takes about 2 hours. Since yours took much longer may indicate that its battery has reached its useful life ... and time to consider getting a new mouse.