Sorry but it is never that simple!
I don't think just asking someone out makes them a gf/bf - there needs to be some time spent together to get to know each other and see if you are right together first. it might frighten the other person off if they just want a date to see if they like you, they think you want a proper relationship starting immediately!
It's always going to be different, depending on each others feelings for the other person. Some couples "click" very quickly and would very soon be calling each other bf/gf - maybe after only 2 dates.
Others might still be "just seeing each other" after several months. Putting on the bf/gf lable can put a lot of pressure on someone and makes the relationship sound serious, and sometimes it takes a person a while before they are ready for that.
it is probably not a very good idea to classify a relationship on the type of kissing/physical side of things. A long passionate kiss doesn't mean a thing if it's at a party between two people who don't really know each other and have been drinking all night! However, after a few good/successful dates, you are obviously more than good friends and the peck moves onto a proper kiss, that might mean the relationship is turning into bf/gf mode.
The worst thing is if one person is just after the physical side/sex (say the guy) and so he wanst to start kissing the other person very soon, but the other person (say the girl) is more interested in the emotional/love side of things. That would make is hard for the girl who thinks that the guy really likes her and starts thinking of him as a boyfriend, when all the guy wants is "one thing"!
I think you will know between yourselves, if it feels right to refer to them as a bf/gf, then it probably is. You could even try it and see - e.g. let them hear you on the phone to a friend that you are going out with your bf/gf tonight, and see how they act. if they smile, it's obvious they think of you the same way!
If it still seems a bit early, even if it has been several weeks, or that the other person doesn't seem ready, maybe you are not quite at the stage yet.