I think 20 hours is a bit of an over-exaggeration. Maybe 5-6 hours if youre really hell bent on getting all 27.2 with stamina down. But for someone content with 26.3s or Kin/beast down curses. Now getting to the point of being able to even farm then is around 5-6 more hours of chalice dungeons. And thats if you know how to speedrun them.
It's all RNG-based, it can even take exactly 3 tries if you're the luckiest person on Earth, or it can take 100 hours if you're extremely unlucky.
On my last character (the Lady Maria cosplay), it took me about 20 hours, but I was watching YouTube/Twitch the entire time and I took a few breaks here and there, I can't imagine sitting there focusing on Merciless Watchers for anything more than half an hour, so it might take less time if you're really concentrating.
Not that they took that long of a time anyway, I used 2 Clawmark runes, so they were going down with two Viscerals, and the gun guy dies with two full stamina combos, they can be beaten in 1-2 minutes at most, usually less.
Bowblade with concentration on BT is basically just a better gun build. Very viable in PVP and if you have decent Skill you can do decent damage with that and then finish enemy off with arrows.
It's much higher than that, I used it on one of my skill builds with minimum Bloodtinge and it was doing very good damage with base level gems, at +10 it has S Bloodtinge scaling, so I imagine it can get very powerful.
To be honest there is very little farming required for good gems. I have bunch of +20% - +26% gems with decent curses and I have only done one Root Chalice. Having all gems to be +27% doesn't seem to be worth the investment since we are talking about >100 damage difference per hit at most.
The difference between 20% and 27.2% is HUGE, it may not look that way because the difference in raw percentage numbers is small (20 to 27 does look small on the surface), but blood gems stack multiplicatively, there's a reason people go through the trouble of getting the Lost Chikage because of one circular slot giving you the possibility of increasing blood damage % to 31.5, and that's just ONE slot.
I can show you in pictures form how much of a difference it is, but I'm not at home right now.