It's based on a few things for me on whether or not I keep reading..
First hurdle's the art-style. Comic thumbnails might be a little deceptive on a few (as in colored covers but black and white illustrations), but still, the chance will always be there to keep reading. I'm admittedly a little shallow in this regard, but if it doesn't grab me on visuals, I probably won't read it.
Second one's the summary/premise. It better be basic and appealing enough! I've been getting awfully busy lately and time's valuable, so complex stories from the get-go are a nope for me. I don't wanna be scratching my head and wrapping my head around it after the first update and re-reading it tbh.. it needs to start simple enough for me, and if it gets complex later, then that's cool!
Third one's less defined, but it's size and quantity of updates. Single page updates are very unappealing to me, but I'll still give it a chance, as long as they come out often enough. I let 'em build up until I feel like there's enough to keep me occupied a while. Longer updates with multiple pages are easier to judge as they're put out in a way that tells more of the story, so in this regard, I give it about 3-5 updates before I'm out for sure.
This fourth one doesn't happen much, but there's been times where a premise is super interesting but the start-up is disappointing for me. In these occasions, I stay subbed, but I stop reading until it's much deeper in its run. At that point, I'll dedicate a bunch of time down the line to just trying to read it through. At this point, it's more like a few slow chapters before the good stuff finally happens like I was hoping to see from the start. It happens, not every story's gonna grab you immediately, but the promise is there.
All in all, the first two hurdles I mentioned have to be cleared for me, otherwise it's a pass. A few long updates or many short ones in one go are my jam. But those rare ones that I have high hopes for that fell flat at first will get a super generous second chance later on. If it doesn't grab me after that then yeah, byeeeeee~