The rpg books give no reference for how long a turn is for thaumaturgy, even when just talking about adding power to it each turn. However, Dresden is able at a few different times to quickly chalk out a circle - we get the impression that it takes him less than a minute or so to do so.
So it really depends on how long you are equating a combat action to: if you see a combat turn as 6-10 seconds, like most other games, then you could be looking at 6-8 actions to get thaumaturgy spell off in a minute, which is plenty of time to get dead in.
I'm kind of on the fence, here... I think it should be kept separate than evocation (not instant), but that 5 minutes is way too long to really represent what Dresden does. Then again, whatever you choose will also represent how long it takes him to make a potion, which always takes Dresden hours... so a perfect representation is impossible.