Once the original container is opened and sealed, the absorbant will react with the minescule amount of CO2 that it got in contact with. There is very little in normal air. Once that is absorbed the process stops.
To be honest I'm not sure how long an opened and resealed container could last ... just gotta make sure you dive enough so it won't expire.
That's really hard to answer as the preparation time varies considerably from one unit to the next. Just take charging the scrubber. The O2ptima is probably the quickest, using the Micropore cartridge instead of granular absorbant. Probably a minute or two. Filling the Sport Kiss scrubber takes me about 5 minutes, filling the PRISM scrubber about 20 mins.
Getting the PRISM ready to dive takes me about twice as long as the Sport Kiss, about an hour.
Then again, the PRISM lasts twice as long as the Sport Kiss, too.
I usually take my time and do it slowly and thoroughly. Doesn't seem to be the place to save a few minutes, and I enjoy doing it. Part of RB diving.
But the general setup is basically the same, fill and analyze tanks. Fill scrubber. Assemble unit and check for damage, fix or replace parts as necessary. Check valves, check loop integrity, check cells and calibration, electronics where applicable.
Immediate pre-dive checking valves and loop integrity again, gas content, breathing off the loop to check scrubber and warm up absorbant, raising pO2 to 0.7 ata. About 15 mins, during some of which (pressure tests) you can get your suit and ther gear ready.
In the water bubble check to make sure there are none.
Jeff Bozanic's "Masterring Rebreathers" has pre- and post dive check lists for a bunch of rebreathers in the appendix, including filling isnstructions for the scrubbers. Highly recommended reading for anyone intrested or diving a rebreather.