Two to three hours. "You do this throughout the ms." is what I write the second time I encounter a problem, and then I quit marking it. If I have to say that too often, I'm done after Chapter 1. The author should fix those errors throughout before finding a second beta, imo, if s/he didn't know about them before today. If s/he did know and didn't fix automatically while writing or editing, that lack of care and professionalism at this stage bodes ill for a professional career and I'm not going to play the game "serious writer" with them when they so clearly are not one.
I'm a tough-sell beta, too. (Though I did one of Chris P's stories from here when I was new, and that was a delight, so I'm glad I offered). I look at most of what's up on SYW and think, "nope, no way, never would beta for this person. Never ever ever." It's rude to ask others to do your basic high-school-level corrections for you. You should do every single thing you can to make a ms. good before showing it to a stranger and asking them to use up a portion of their life on helping you. For free. (And even a decent SYW offering where the person didn't give me a "thanks" for taking my time out to help them/encourage them makes me doubt I want to beta for that person. If I'm going to do unpaid labor for you--and your mother should have told you this when you were but a child--it's polite to say "thanks.")
I was going to put up a beta offer when I was done with my WIP draft, but after seeing so much slop on SYW, I've decided against it. Sorry to be blunt, but I'll pm you if I think you're worth my time and are writing in a style/genre I like. (and if you never put up SYW, as I don't, we'll never know about each other, which is also okay. No doubt we each have other beta options.)
I digressed, didn't I? 2-3 hours.