from JAR-FCL
(2) Co-pilot flight time
The holder of a pilot licence occupying a
pilot seat as co-pilot may log all flight time as
co-pilot flight time on an aeroplane on which
more than one pilot is required under the type
certification of the aeroplane, or the
regulations under which the flight is
conducted.
By regulations one assumes they mean statutory regulations rather than something you, your insurers or your flying club have thought up. Work out whether a co-pilot is required and you have the answer.