There are no different rules relating to the validity of medical certificates that apply solely to people with learning disabilities.
The MED 5 certificate is a ‘special statement’ given by a doctor, where they have not seen their patient, and based on the report of another doctor, for example a hospital doctor. The rules governing the provision of a MED 5 certificate do not allow it to be issued for a period of more than one month.
However, a MED 3 certificate, the ‘usual statement’ can be given for longer periods; after the first six months, this includes ‘until further notice’.