I can figure out pretty early whether I would be able to stick with someone long term or not. Whether the reverse is true and they'd get sick of me is up in the air. But if I'm relatively certain that I'm not going to change my opinion of the guy at any point in the foreseeable future, I'm game.
That is, however, taking into account that I own my house and have a spare room that he could use if he wanted a break or things were to go sour and he needed to stay until he could figure out an escape route. My house is big enough that we could easily live together and not really even need to see each other on a daily basis.
Pre-homeownership was another story. Would not have even considered getting a place together or shacking up with a guy unless marriage was already in the works. Too many what ifs, and too big a headache if it didn't work out.