I don't use cartos, but I've had some issues with the Tobacco/Menthol as well. I use
highpings atty cleaning method, but I pre-soak with denture cleaner first. Getting the atty as close to "as new" condition seems to be the key. Word of caution, though: it's easy to fry an atty with this method if you're not careful.
Using a new atty, my flavor starts to drop off within just a few hours. After that I can usually continue to use that same atty an additional 12-24 hours before the build-up becomes extreme. I've tried the Tobacco/Menthol in both 901 and 510 attys with pretty much the same result. I've even tried mixing in a little PG to try and lessen the speed of the build-up, but I couldn't detect any difference.
I've lost a few attys trying to burn them clean, but all in all I just consider it a cost of doing business with this particular e-liquid. I love the flavor and to me it's worth the extra work and expense to keep using it. Currently I'm trying out one of the newer XL/Mega 510 attys and while it's early yet, the flavor seems to be holding up longer (almost 48 hours now and still getting good flavor/vapor). I don't know why the Mega atty would hold up any better than a standard 510, but I'll continue using it and give a more informed evaluation after more time has passed.
Edit: Mega 510 atty #1 stopped working this afternoon... it just "died". Could have just been a bad atty so I'm on to Mega atty #2 for further testing...