Well, according to RFC 1035 (the DNS one), 2.3.1, labels (that is parts of domain names separated by dots) may not exceed 63 characters in length. 2.3.4 elaborates on the limits a bit; the entire hostname may not exceed 255 characters in length. It’s entirely possible that some registries apply further limits on name length, but the above is the bare minimum (well, maximum) of what is required by the DNS RFCs.
As to the OP : I have been using my real name in my domain/email address for years. If you communicate with people using your real name, there is really no difference between doing it as JohnSmith@gmail.google.com or John@smith.org. If you have any domain at all you are probably going to have your address public, as well (though “privacy cloakers” such as DreamHost exist; you can cut down on the spam to the hostmaster-address somewhat doing that, but other than that it’s really not that useful; don’t believe for a second that they are going to keep your details confidential in face of a subpoena, for instance).
(and if you are looking at not changing your mail address for a long, long time, you should probably plan on getting really strong, strong filters. The amount of crap being slugged at your address increases every year