Hey Shamasa,
I talked a bit more about that in the last course that I launched yesterday with the Top 10 USDCAD EAs. Really there is no precise answer to this question.
If you test the EAs too long on a Demo account, you will see the profits in the Demo account. Every strategy has its profitable periods and losing periods (you see that in the equity line of every strategy). So if you test it too long, you are in this profitable period for the strategy, and when you decide to move on live, you might hit that losing period.
Basically the more you test on the Demo, the closer you are getting to that losing period.