The key statement is that warnings are available 12 seconds after the thing starts scanning - which it does automatically when you set takeoff thrust. Since this can be too late for a timely warning at low speed hit WXR as soon as you line up, then the radar is fully operational once you need it.
I fly the Classic so I'm not entirely sure, but I seem to remember reading in the Bulfer guide that with PWS enabled, there are four radar sweeps of 4 secs each, something along the lines of WX capt, PWS capt & f/o, WX f/o, PWS both again. So it takes 16 secs to complete the cycle, but PWS can be detected on the second sweep (max 8 secs).
So I'm wondering if this 12 secs relates to how long the F/O has to wait for the WX paint on his side (if PWS is active) and not to how long it takes for PWS to be effective?