I do not think there is a real set amount or at least that anyone has tested.
In order for crit to actually make a difference you would need a high enough number but at the expense of possibility other stats.
Remember Crit= % of actually happening, it is basically RNG, now with spell speed it is guaranteed and you can fit more fire into your rotation.
At 1003 Crit i would actually focus more on SS now, 1008 is not bad by any means but myself i prefer in the 1100+