It depends on a lot of variables for me. If the directing, grammar, etc. are all bad, that’s they type of story I’ll quit within the first episode. But if it’s only one or two things that are clearly just a mishap (for example, a scene were the zoom is off but everything else is fine), I’ll continue.
If I’m struggling to enjoy the plot, I usually will give it three episodes to really decide. But mostly what happens is I’ll find a story description that really pulls me in, but when I start to read it I’m just not connecting, so I leave it in my favorites and plan on returning to it when I’m in a different mind set to see if I feel differently about it. So I guess I never really fully quit a story.