10-11 minutes, but you shouldn't be aiming to kill your opponent - rather to force roaches and secure your third for the transition. Having a skeletal ground force makes you far too vulnerable to just being killed by mass zerg ground, and you want to fill zerg's supply with supply-inefficient units that can't kill Carriers.
Really, you could choose any form of midgame pressure (5gate robo/6gate/6gate robo/blink/etc) that creates such a situation.
Even the smallest donations help keep sc2sea running! All donations go towards helping our site run including our monthly server hosting fees and sc2sea sponsored community tournaments we host. Find out more here.