My rule of thumb is any time I'm diving deep (>200') I start with a fresh pack, but I'm comfortable with re-using a scrubber for two days in a row for shallower dives given certain criteria.
Any time I've put more than 3 or 4 hours on the scrubber, I bin it, period. I have put over 7 hours on these scrubbers (multiple times), but they were single dives with the majority of the time (more than half) being on deco. Any time I'm over 3 or 4 hours, I consider it cashed enough that I have zero interest in re-using it.
Any time I'm debating about starting with a USED scrubber and I know my dive plan will get me above 4 hours total on the scrubber, I bin it and repack. So basically, my rule of thumb is roughly 2x 2 hour dives and I'm comfortable reusing a scrubber two days in a row, after that I'm dumping it and starting fresh.
Basically this ^^^^^^. 5.5# in the SF2 and 7# in my Pelagian. I had the 8# radial in my Meg before I sold it, but never stretched its legs that much. If it's anything deep, it gets a new scrubber. Sorb is cheap, funerals are expensive. I tend to estimate 1lb/hr as a conservative baseline. Obviously depth, temp, etc. play a role.
If I put 4 hours on the SF2, I'm dumping it, because then next dive isn't gonna be worth it unless it's a super shallow rec type of dive. Then I'd limit myself to an NDL on a typical reef bimble. I wouldn't put it in a cave simply because the risk profile is greater. NDL in a shallow Mexican cave and NDL on a 15m reef are very different beasts. If I do 2 hours in the cave, I'd do another 2-3 in the cave. I'd do 6-8 hours total on a single dive provided a significant portion is lazy deco. I wouldn't roll 8 hours in the back end of Naharon since the deco involved is mostly nothing, and the majority of it could be swimming.
If I put 4 hours on the Pelagian, I'm good for another 4 unless it's deep. I'd do 8 hours on it without a second thought. Again, single dive limits are different than re-using the scrubber, and similar rules apply, but I'd give it another hour or two roughly on each simply because of the size of the scrubber.
Bottom line, I plan for a pound an hour swimming. I'll stretch it if deco is involved. If I'm going deep, I bin it and start fresh. Sorb is cheap, funerals are expensive. Easy rule to go by, and has served me well.
I seal the SF2 scrubber in a dry bag, and the Pelagian I've got a couple port plugs that I pop in the unit since the scrubber isn't removable. I pop the bottom off to get rid of any moisture, then seal the whole thing back up.
Unrelated but related, I'm thinking about building a radial for the SF2 just to give it a shot. I'd love it if the little Meg 5.5# radial would fit with an adapter but I don't have one to try. Anybody got a used radial they want to get rid of?