I asked Chris Simmons about this very topic. If you're not familiar with him, he won both the NRL22 and PRS Rimfire Championships last year.
He detailed how and when he cleans his rifles. Usually, he cleans the night before a match so that when he shoots sighters the next morning, he'll re-season the bore and doesn't have to worry about how many rounds have been shot since the last cleaning.
He said his theory is that at some point, accuracy will drop off. The question though is at what round count after cleaning does the accuracy drop off...
If he cleans before each match, he's consistent.
I use poly-pellets soaked with Bore-Tech Rimfire Blend. As others have mentioned, a bore guide and a one-piece rod are important as well.
I usually will place 2 - 3 soaked poly-pellets at the bore and another 2 - 3 in the chamber and throat area. I let those sit for 30 minutes or more and this seems to do a great job of keeping the carbon build-up to a minimum.
As always, YMMV
