The max length for non-IDN domains is 63.
There are 63 characters allowed in these domains:
abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
and -
There are 63! Possible combinations of 63-distinct-character domains. One example is
YMfIHcoStbLKkvORZiTWwanQr-mlhXzJgFuUPeCdyxqNjBpGAEDsV.com...