I always do every side quest in C6 and, in all of my 12 playthroughs, none of them came close to 20+ hours, I'll give it 6-8 hours top because I usually finished C5 and C6 on the same day, with all of the sidequests and characters' quest, and even got supplies from all of them, and all of the none buggy CG, and maxing the influence with all of factions, every single playthrough. Unless, of course, you meant trying C6 with every single main quest route in one playthrough, then maybe, but I already did all of the main quest route, only in different playthroughs so I can't really count that. It's not like you have to anyway, I only do Hacker when I play Alf Britt or Persuasion when I played Seidr Britt, and trading maps (which I'll admit take longer) with Valkrye (the quickest way is just brute force). But most of the time I spent was on the battle itself.
Granted do, I stop caring about stacking resources once I understand how each "classes" work, Valkrye, just stack valhalla since earlier chapters and in C6 just focusing in processing war supplies in between battles, and Seidr is always the easier, I never had problem with stashing in Erland even with Seidr character because I usually stock enough from C4, you can get a mass amount of compost for valhalla enzyme and a lot of woods in the Valley. So I just breeze through C5 and C6.
C6 is easy once you understand the mechanics of each class and the resources they need, and because it is the most combat-focused chapter, you'll get an advantage if you stack resources in Erland's stash (mainly valhalla). Always stack valhalla whenever you can. Then I just breeze through everything because the items for one quest you'll get from another quest anyway. It's especially easier than C5 for Valkrye build because processing war supplies are free and you don't need war supplies to do so, so I'll just stack on Valhalla, process war supplies in between, and continue on fighting. Seidr doesn't need power so use all of the wood to get incense and Valhalla serum, and I usually stack enough Potent Huldra essence from earlier chapters where resources to make them much easier. C4 and C5 you're rolling in money, literally. Alf is the hardest because you need power, and it was hard to make power in C6, but once you have enough, like 10-20 power, even less if you have good luck, one battle will enough to get you stacked with power. I think Alf is always the longest for me to take C6 because I got down to around 10 or 9 in the countdown when I managed to finish main quest, not counting the asking for supplies.