It’s been a while since I’ve read them. But for “hours”, I’d go with the following (all assuming autoplay set to change text upon completion of spoken lines):
Air: I don’t read Japanese or broken machione translated English, so…
Planetarian: I’d say between 5-8 hours. Not too long, but not exactly chort either.
Tomoyo After: 30-35. I started reading at my own pace halfway through, but I’d say that’s a safe assumption
Kud Wafter: See Air, minus machine translation
Rewrite: 111 Hours. I recorded it for a friend who didn’t have a PC at the time so he could read it on mobile.
Harvest Festa: 20-25 Hours. Assuming most lines are voiced, as it’s scripts are roughly 1/5 the kb’s as Rewrite