What is the minimum time to closing SDV?

As far as I am aware there is no standard whcih addresses this. A rule of thumb dictactes the valve should start to close within 10s of activiation. And should go from fully open to fully closed 1-2s for each inch of valve size.

Check API 14C Appendix C 2.1.4 which states the time for any safety device to effect a platform (or by extension system) shutdown should not exceed 45s.

Hope this of some help!