Q: How long the block should I use for deceleration switch, the CNC lathe equipped with GSK 980TA1 CNC control and the ball screw lead is 10.

A: 25mm block will work.

GSK 980TA1 Specification

http://www.cncmakers.com/cnc/controllers/CNC_Controllers_for_Lathes/GSK980TA1.html