How long does it take to be an ERP consultant?

It depends on your educational background and also on the stream which you want to choose in ERP. There are two streams in ERP:

1) Technical

2) Fucntional

Suppose you are from Non-IT then to become a technical consultant it can take6 months to1 year but if you are already from IT back ground it can take less than6 months to achieve a beginner level. To become a functional consultant it can take more than one year.

If you are from accounting or commerce background then to become functional consultant it can take6 months to achieve a beginner level. But if you try for technical consultant then it will take more than a year.

It depends on which track you use and It will take at least one year to  become a functional consultant in my opinion.

Also the implementations you participate and the team you work with will affect your proficiency.