I have a batch that has been in the primary over a week. Its per Jack
Keller, and his recipe gives a number of days after which to move to
secondary. The ferment is noticeably slower (in terms of PA dropped)
than usual for me in one week, because of the cold conditions, despite
heating belts.

My question is, should you move from the primary to the secondary based
purely on days since begun, or should you move based on PA/SG?

My must is very dark purple and daily stirrings are not giving huge
clues as to how much lees is accumulating.

Thanks

Sean