How to maximize Zergling/Roach army by the 12-minute-mark?

Since Stephano is famous for doing the 12 minute roach max, i would suggest watching his stream and trying to copy his play. He perfected that strategy to a point where 12 minute max is quite late for him (11:04 with harass is his record).

Also, you might want to check out this TeamLiquid Thread for more in-depth info on how he does it and how to get there: http://www.teamliquid.net/forum/viewmessage.php?topic_id=324733

Assuming perfect play and only drone production, its about 11:30, Ret did under 12 minutes in a professional game against a FFE Protoss, so its actually a useful strategy to know, because being able to max out fast puts pressure on your opponent.

As for how its done:

Some variation of http://wiki.teamliquid.net/starcraft2/3_Hatch_Before_Gas_(vs._Protoss)

The jist is to drone up to about 50 drones, whilst building up gas, then on 3 bases pump roaches.

Injects need to correct to the millisecond. Minerals should be glued at 0. Take a few attempts to streamline the build, when to take the third and when move to 6 gas