Install requirements and dimensions for Kinman Hx Strat pickups
There should be a minimum of 25.5mm (1.1 inch) between the bottom of the unfretted (open) E strings and the bottom of the cavity, as shown in the diagram below.
As a rough guide ONLY this translates to cavity depth of approximately 17mm but this depends on bridge set-up, neck angle and a number of other factors so cavity depth is not the sole determining factor. The best method is in the first paragraph.
This will provide a gap of approximately 2.5mm between the strings and the top of the magnets when fretted at the 12th (provided the guitar is well set-up). This is a good place for the pickups to be anyway but if the cavity is deeper then so much the better as even more adjustment of the pickups is afforded.