Good grief, of course its cheating! You have to do the whole thing in a certain time - that's the whole point! And the nav exam isn't just testing your theoretical knowledge - its also testing whether or not you can apply that knowledge realistically.
But look, its not that difficult if you are careful, methodical and check everything twice. And actually there is plenty of time.
If your instructor says you are ready, then you are ready. If not, then you certainly aren't ready.
Good luck
H