With all the info (books, articles, and online forums) you can figure just about everything out and make the jump as soon as you want to.
That being said, I made ~6 batches with extract before moving to AG. I thought this was about right because it gives you a feeling for what the specialty grains and hops do before you add all the variations of a mash. To put it another way, all grain introduces a lot of other parameters into brewing. For me it helped to understand by first tasting the outcome of recipe variations, so I knew how to tailor a beer's taste before I got into the complexity/freedom of all grain.
Just one person's opinion...