You always count based on the sum of skill+dice+modifiers (like invocations), so I suppose that would be "before" in your description. The only thing a Power bonus does is give you higher effective Conviction, it sets the amount of power you can use without taking extra stress higher.

Some DMs take offense to the [-1]upgrade making it Weapon:4

Potions take time and resources each time you want to make them, while an enchanted item has to be recharged.

It's the difference between buying more ammo for a gun and having to build a new gun from scratch every time you wanted to shoot someone.

But it would well be within the bounds of the setting and I would argue even well within the supposed themes of the DFRPG.

I'm going to go with "As long as it takes". Which is about the best and the worst answer I could give, I assume.

Let me elaborate. The time it takes to cast a spell is not always an important factor. If you've got the afternoon off, it doesn't really matter if the spell takes 2 seconds, minutes or hours, you'll have easily finished it in the time you have.

For casting a spell when you are under pressure, there are already rules for that. Just gather the shifts you need over the course of a few exchanges, and as long as you aren't interrupted or fudge up the roll, your spell works as planned.

For the SK spell you talk about in your link, the casting process doesn't even have to be part of the equation. The GM compels the "mind fog" scene aspect on Murphy and Harry. Murphy's player doesn't want to go to lala land, so she decides to buy the compel off. The GM asks her how that happens, and she says "Well, can't our wizard here do something?", and they decide that is a good idea. Harry's player doesn't have that many fate points to spare, so he accepts the compel. He'll still be able to fight against the mind fog invading him, but at times, that's going to take his full concentration, so he'll be way less effective in the ensuing conflict. Hell, he might even do something stupid like throwing marbles on the floor in order to stop an ogre.

The things that take time for any thaumaturgy spell is the preparation. Getting the right ingredients can be a session on its own, if you want it to be. Preparing the circle, making arcane calculations, ritual cleansing, meditating and what have you all (can) take time. But it always depends on what kind of spell you have, too. For Harry, it makes sense to have a quick tracking spell, that's his bread and butter, so he can do a spell like that with a rough chalk circle and a piece of string. Other wizards might need a lot more to do that, but they can make a tree grow with just an effort of will, for example.