In my eyes, Ron and Hermione always had a wee bit of a ... soft spot, for each other. Like, they could be completely annoying, but they wouldn't be able to go on without the other. I think they actualy fell for each other at different times.
Ron-i would say he sort of got a crush on her in 2nd year (Because everyone was all feeling protective of each other, and he probably thought it'd go away). He probably felt a bit of a pang when he saw Hermione in the hospital wing. He also probably liked her, because well, she was his best friend, and basically the only girl, apart from Ginny he was close to. If i was him, i'd have a crush on her, which sounds a bit weird coming from me ...
So he probably started liking her around then. But thought it went away after a while. But you saw (er ... read) that awkward hug on PoA. He probably realised the feelings
hadn't gone away then.
GoF, Ron was jealous of Krum. That was when he realised without a doubt that he fancied Hermione, because he didn't like seeing her with someone else (and that situation only has one explanation really). I'd say he realized it was luuurve ... after the whole Lavender fiasco. Yeah.
Gosh, that was just Ron ... Hermione i see falling for him a bit later. Probably thrid year was when the stiring feelings came. She was more upset about Ron hating her than she should've been, and she didn't know why (this is me just speculating here). When she hugged him, she wondered why Ron was being all awkward.
4th year, after the whole Last resort thing, she realized she liked him, i'm guessing. When Ron went out with Lavender, she was obviously hurt (Because he said
yes to going out with Hermione ... dude, anyone would be upset). When Ron was poisened, was when she realised she loved him.
If there was ever a time they realised they couldn't live without the other after all this, was when Ron left, for Hermione, and when Hermione was being tortured for Ron. Because both times, the other could easily have been killed.
I don't think at first they knew the other liked them. I'd guess after the Lav-Lav fiasco, they both sort of mutualy knew the other fancied the pants off the other, but neither wanted to be the first to admit it.
Hermione would insist on tridition, and Ron's too proud.
But Hermione gave in, in the end, as we all know (And love).
PS. Wow, looking back on what i've wrote, i have seriously rambled a lot. Consider this a recap of "RHr Through the Ages"