You can start testing anytime, because it is always good to see the changes that go on.
It is quite important to test it now actually, to see what the parameters are like. If there's a bit of ammonia, nitrite and nitrate (Let's call them ANN), then you know that your tank is not cycled (of course).
If you see nothing though, or very low levels of ANN, try ghost feeding and see how the tank responds. This is because even if you see nothing, it doesn't mean your tank is cycled (for example, I can fill a tank with saltwater and test it and it would have nothing, though obviously it is not in any way ready for live stock). Ghost feeding will allow you to gauge if your tank is ready to deal with live stock.
If ANN goes up (which it should), and stays high, then you know that your tank isn't ready yet. But if ANN goes up then down significantly, you know that it is actually cycled - as your tank is able to deal with waste produced.