|www.rodneybeede.com||"I would love to change the world, but they won't give me the source code" - unknown|
253 characters. The encoding requires the length of each label to be encoded (typically where each . goes). The first label and the root (end) label take 2 octets from the 255 octet (not character) maximum. US ASCII leaves you with 253 characters (including the dots in the middle which are lengths).
Of course if you have a DNS entry in UTF-8 then things are different.