If you look at the discharge curve of a ni-mh battery you will notice that for 90% of the cycle it is disharging linearly. The last 10% 5% the battery ( for regharging purpuses) would be considered discharged.
It would be unwize to compleatly discharge any NI-mh battery.
Sorry , confused you , actually , I meant when to stop discharge for safe purpose , to prevent overdischarge. I'll take Ante's answer , thanks you all.