90 seconds is a good time. From my experience on cs:go servers, the wardens are slow most of the time.
Example: Warden: On my go, Take one step out of your cell and afk freeze.
*~1 min goes by*
Terrorist assholes that mic spam: GO! GO! GO! GO! GO! GO!
Warden says during the middle of this chaos: go!
Terrorists still saying: GO! GO! GO! GO!
Warden says: MUST BE DOING THIS BY NOW!
*Mass genocide*
The point is, is that a longer mute time at the beginning of the round will cause much less tarping. (horrible example)