I would like to expand on the comments from Kriki and Mattrax.
C/AL is very easy to learn, and if you know pascal it would not take more than 3 weeks with proper instruction. If you know access or VB then it will be harder since you will have to go through a bit of an unlearning curve and the usual fighting of "yeah but why does it do it like that".
Whilst this means people can be up to speed with C/AL very fast, it gives them a false belief that they are then Navision programmers. Much like someone that knows how to use a scalpel to dissect a frog thinks that they are a great brain surgeon.
The key is to learn the Navision application, and the only way to do that is by using it. Unless you can post an invoice you can't write code to add a new field to an invoice, and you can't make changes to reports, because you will make the wrong changes.
So your first step is to forget coding, and instead work with the users. You have to be able to do the job of every user in the company, order entry, posting, accounting, journals, adjustments, financial statements, collections, physical inventory, manufacturing, write offs, the whole lot. Not only know how to do these, but know exactly what is happening in Navision when you do this.
That process takes 1-5 years depending on how involved you are with the project before you are ready to start being productive.
