It depends on how well you pick up on things. I take tap classes and learn things very quickly. The hardest thing about tap, I think, is the amount of pressure to put on toe vs. heel and the movement of the leg. (example- a lot of tap moves have your leg bent)
If the class is not strict, you will probably be able to catch on quickly. For your first class, you should concentrate on the teacher and stay in the front row. (You learn easier there) You could also go to YouTube and just search "easy tap steps" and just see what you are getting in to and to prepare yourself.
Hope I helped!
I am a tap dancer!