47 CFR 10.430: Character limit.

A Participating CMS Provider must support transmission of an Alert Message that contains a maximum of 360 characters of alphanumeric text. If, however, some or all of a Participating CMS Provider's network infrastructure is technically incapable of supporting the transmission of a 360-character maximum Alert Message, then that Participating CMS Provider must support transmission of an Alert Message that contains a maximum of 90 characters of alphanumeric text on and only on those elements of its network incapable of supporting a 360 character Alert Message.