How long should I accept a guy dating other people before I know it's just not going to be me?

I have been friends with this guy since August. There was never anything physical b/w us until Halloween-ish. We just kind of went with it, and still are. We like each other and we are trying to be honest with each other. I know he met someone a few weeks ago and is going on a date this weekend.

Of course I don't like it (I keep that to myself). It's not like we are exclusive and he has every right to meet other people if he chooses. I guess I wonder how long does it really take a guy to realize whether he likes a girl or not. Eventually I should be able to tell that it's just not going to be me.

I don't want to be a sucker, but I also want to be fair.