That's the one! I had hoped you'd be along to point this poster at the right info - thanks John :-)
So it is a maximum absence of six months, and this is the regulation:
Article 11
Validity of the residence card
1. The residence card provided for by Article 10(1) shall be
valid for five years from the date of issue or for the envisaged
period of residence of the Union citizen, if this period is less
than five years.
2. The validity of the residence card shall not be affected by
temporary absences not exceeding six months a year, or by
absences of a longer duration for compulsory military service
or by one absence of a maximum of 12 consecutive months
for important reasons such as pregnancy and childbirth, serious
illness, study or vocational training, or a posting in another
Member State or a third country.