I made a simple animation with a box that rotates when you press the “up key”, but I can’t figure out how to make it so that if you hold it down, the animation keeps on going until you let go. Help?

No one can help you if you give that small amount of information. What type of animation? What actuator did you use?

try loop stop, loop end, flipper etc. in the ipo/action actuator until you find what you seek.

yes use loop stop for your animation and it will continue to rotate untill you let go of the key.