The limit is imposed by the server, not mIRC. mIRC has no way of knowing what the cut off point will be as the length is not given as a token in raw 005 by any IRCd I've seen.
Testing the limits once on the servers you use and then remembering them is the best solution here. If you're unlucky then there won't be a specific limit for quit messages and the cut off point will also be decided by the length of your nick!user@host.
