I played TW through once (about 40 hours), TW2 through twice (probably 60 hours together), and TW3 through 4 times (500-ish hours ... maybe more) and about 200 hours of GWENT between closed / open beta / homecoming / Thronebreaker. So ... I would guesstimate 800 ish hours. The only one I actually have a clock for is GWENT.
The Witcher 1: 95 hours in Steam but all together (with the Disc version) ca. 250-300 hours (10-15 playthrough)
The Witcher 2: 315 hours
The Witcher 3: 2450 hours
Gwent: 1957 hours
Thronebreaker: 135 hours
Damn, and i thought i had a lot of time in The Witcher 3...
Witcher 1: i had no tracker, but i estimate 40-50 hours
Witcher 2: never played unfortunately, since i have no X360 and my PC almost died when i tried Witcher 2 on it. Someday...
Witcher 3: 387 hours (i know the exact number because awhile ago Sony sent PS4 players an email showing the game where they had spent the most time, and mine was TW3, my favourite game of the generation)
Gwent: 1200 hours and counting.
Thronebreaker: 40 hours
W3 something about more than 200 hours with 100 percent. Really quality time!
W2 i believe 100 to 150 hours(in my 2nd playthrough i believe more than 60 hours with 100 percent iorveth path)
W1 150 hours but most of it running.
Witcher 3 is the absolute record for any single player game I've ever played in 25 years. 600 hours. Witcher 2 is about 100 hours total. Witcher 1 is less than 100 hours. SP games rarely go above 100 hours in my case. It's enough to complete the hell out of them.
The worst (best) part is that there was no grind in those hundreds of hours in Witcher games.