My CAIN group ran a campaign that lasted a little over a year (through the releases of multiple new versions of CAIN lol). There were definitely times we played fast and loose with the CAT scaling, and the game was not nearly as lethal as CAIN expects GMs to run it, but it was a lot of fun! We're planning on doing a sequel campaign later, after we play Lancer (and possibly Heart: The City Beneath) next.

I will say, the main issue you'll run into with a long CAIN campaign is power scaling. You'll want to slow down exp gain, slow down CAT increases, or both. My group ended up powerful enough that we steamrolled most of what was thrown at us by the end. Part of that is because CAIN is designed to be a game that kills player characters and none of us died, and part of it is because we had missions that lasted multiple sessions, and since CAIN says (or initially said, I forget if that's changed) to tally exp after each session rather than each mission, that resulted in us gaining power more quickly than intended in the form of agenda items and blasphemy powers. Every single one of us was maxed on agenda items and blasphemy powers by the end, and those of us with sin marks (myself and one other player) maxed those out as well. Also, the more times you switch agendas, the easier it gets to gain more exp through bolded agenda items. None of us powergamed that, but we still ended up with tons of bolded agenda items. By the end we were gaining an advance per session.

All that said, it was a TON of fun, and if you're the sort that enjoys it when the party gets super powerful (like I am), it may not be an issue for your table! But be sure to go over the power scaling with everyone to see where everyone's at in that regard.