I'm betting they both have feelings for each other in the open by the end of Season 3, don't know if that translates to them kissing or actively being a couple.
I don't think we're going the "Ross and Rachel" route, they're going to go through a lot of shit but they'll do it together. No "will they or won't they?" longer than necessary, no forced romantic drama bullshit with wacky misunderstandings or cheating.
We do have more than half the show left, and really if Nefcy asks for the legendary Fifth Season they'll probably give it to her, but it definitely seems like she knows how she wants things to go and won't drag things out.