My Magic Keyboard is losing charge quickly. What can I do?

If I remember correctly, the computer should have also come with a cable to connect the keyboard for charging. Use it.

I do not know what time frame is "normal" for holding a charge. Personally, I see no advantages to using either a wireless mouse or keyboard, and there are some definite disadvantages such as when you have to log in at startup and the computer does not recognize the keyboard. I find that using wired devices serves me well and they are not all that expensive these days.