253 octet
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... NAI length of at least 72
octets. Support for an NAI length of 253 octets is RECOMMENDED.
However, the following implementation issues should be considered:
...
... User-Name
attribute may be included in a RADIUS message and the maximum
attribute length is 253 octets; RADIUS is unable to support NAI
...
...
o A recommendation to support NAI length of at least 253 octets has
been added, and compatibility considerations among NAI ...