thanks everyone for the replies. we've been sticking to 4.5 hours in the cold waters here which can get in the 6 C/ 43 F range in the winter. I follow these rules of thumb:
- before a deep dive, fresh sorb (thought I don't really dive deep yet)
- deepish dive, then progressively shallower dives follow
- 4.5 hours in cold water

Only 10 days till our Bali/Alor trip
I'll report back after that... planning on 6.5 total hours per fill.