Not 100% sure, it depends on the playstyle of the zerg. When do they stop droning? When do they add spines? etc.
Off two base, the earliest is around 10-11 minutes (I think), but this should be easy enough to scout and respond to since he doesn't take a third base (he could also be doing some infestor thing but that's less likely)
Off three base, you have a couple more minutes. Most players don't go muta off 3 base because if you do any 2 base all in as protoss, you win.
id say if you scout a spire prior to the 10 Minute mark you should make a push to force tgem to spend stockpiled resources and larvae on other units. this will drastically delay mutas and muta count. if muta is out prior to that just steamroll their base as there is no possibility of them having an army to contest against yours. i still dont understand how people can lose to mass muta. my experience anyway
If you ffe. they should fast 3 base by 4:30 if you don't see this and they have snipes up at thier natural its 2 base muta thats the fastest actual build for muta. 3 base ling/roach into muta comes after if you open stargate void phenox phenox phenox phenox phenox should go down by the 2nd phenox but if you scout after your harrass you should see it before they pop. These are my timings anyway.
Pretty much what everyone else said, if its 2 base around 10 mins, if its 3 generally aroudn 13 mins or so but can be quicker if you yourself are taking an early 3rd base, sicne he wont feel pressured to make roaches to defend a possible all in the mutas do come out around 12 mins then (atleast for me as a zerg player)
From the normal FFE -> 3rd base, and you see 3rd base from zerg - if you take your third base and are not pressured significantly, you should be prepared for infestor or muta tech - you can scout it but just make sure your units are positioned well so the first wave of mutas don't do a dicktonne of damage
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