RFC 1035 Section 2.3.4 specifies a FQDN maximum of 255 octets, which for practical reasons, translates to a maximum of 253 ASCII chars.
UTF-8 is now also permissible for hostnames, but because of variable length character encoding, it's not easy to say exactly what the maximum UTF-8 chars would be (depends on the chars used in the hostname).
In any case, no matter if ASCII or UTF-8, a 100 character hostname should not be the maximum, according to the relevant RFC.