Set the beginning rotation to whatever you want to have, as example I started at X 0
And the end rotation to for example X 360 or anything 360*n for multiple rotations
Because a full rotation is always 360°, you basically just always add +360 onto your starting rotation

Make sure that the keyframes are lineare so they rotate on a constant speed and loop seamlessly

I have not done any rotations on mixed axis, like something rotating on a mix of X/Y so i can not 100% help with that atm if you are trying that
also can't look into blockbench rn since my PC broke down
If you are looking for a work around for a mixed axis rotation
1. Align whatever you want to rotate on facing one of the axis
2. Have it rotate like explained at the very top
3. Create a new bone to put the rotation bone into, and use that new bone to align it with the model the way it was intended to be

I hope it helps?
If not I can try rewriting it or even ask someone from my group to set up an example model and send it to you via blockbench web