Setting a 'Maximum length' to a CCK field isn't working

Hi,
I have a field called field_description which I've set a maximum limit of 120 characters. If I go over this amount, upon submitting I get the error "Description is longer than 120 characters."

What I want to know is whether there's a way to just stop the user from typing any more into the field once they've reached the 120 character limit rather than have to submit the form to find out they've gone over and keep guessing and submitting again.

Any help would be really appreciated.
Cheers.