My personal opinion is that you should try and finish CAS altogether by the end of your first year in IB. In your final year you have so many other tensions like finishing IAs, commentaries, EE, WLs and so on and so forth, you rather not have CAS looming over your head as well.
At my old school, they cut off the number of hours you could get from a particular activity to 20 hours/per year/per activity. So you should organise your work around that. I joined two NGOs and worked for them getting 20 hours each for Service and then did odd bits and pieces of work around school, like whenever we had interschool events and such and scraped together another 10. For Creativity I had no problem at all because I was doing Theatre/MUN/Newspaper and a load of other stuff, so I had about 150 hours of Creativity alone. Finally I did Action by joining two school sports' teams (Wrestling and Rugby) and got 40 Action hours there and again with hiking trips and snowboarding, got another 20 hours there.
When I finished the entire program, I had 280 Creativity hours, 100 Action hours, and 180 Service hours. Just join a bunch of stuff and keep recording whatever you're doing.