you can register a domain up to 63 characters. http://www.quickonlinetips.com/archives/2007/03/worlds-longest-domain-names-63-characters-maximum/
There is a limit of 63 characters but I don't get the logic of registering a very long domain which is prone to being misstyped. If you are just asking of then it's 63. But you should do away with registering a very long domain.