Thaumaturgy in combat "should" be allowed, we've seen it in the books and makes sense mechanics wise. As for how long it should take is a bit rougher. I use a set up found else where in the forms as a base.
1:Prep work: Just because you HAVE everything you need to set up the spell doesn't mean you have the spelled laid out in front of you. Look at the loupgarou fight in Fool moon, Harry has enough time to explain the simple concepts of the thaumaturgy after getting all the items while setting them up.This should be an exchange by itself easy. Remember you need a link to the target that, for all likely hood, needs to be found and use, add a second exchange.
2:Making the mental construct: everything's laid out now you have to focus on what you need the spell to do, form a container to pour your energy into. Add a another exchange.
3:Pour the power : use normal rules here
4: Pull the trigger: at the end of this round you let the spell go. From above you've added an extra 4 exchanges into the equation if you have to.
Now, depending on how fast pace the action is (a knife fight in a phone booth at the far end of this) you might say every two or three exchanges of everyone else counts as 1 exchange for the wizards, but, for the most part, even the most rushed gun fights and what not is a game of cat and mouse trying to get the other guy when he fowls up and a one to one ration isn't off the wall.
As for magic circles I usually make them thresholds with their power equal to the characters conviction or discipline, with a few declarations and aspects the creator can make (usually just "magic circle" for wizard types but for mortals they get, "Empowered by my blood" or something {at a single stress}, giving them a +4 boost)