In some Muay Thai schools there are in fact Beginner, Intermediate, Advanced.
As for how long you become intermediate/advanced is totally up to you. If you put in the hard work, then you'll move "up" the ladder pretty quickly. Try to train everyday and master your basics.
Doesn't really matter if you're a beginner..everyone has to start somewhere. That's just like asking How long until I get my black belt? You don't think you just do.
Have fun
