My way would basically look like what Taran wrote, only without the added duration. I would just focus on the time it is actually used, which is the duration of 1 scene, which is (roughly) 15 minutes.
I would, however require you to either spend fate points or spend tags in the form of valuable aspects. Want flight? Get an ingredient for your ritual that will reasonably grant you the ability to fly. The feather of a gryphon, the heart of a wind elemental, that kind of thing. It should be hard. Hard enough to make other options more viable. In my opinion, you either take powers you want permanently, or you have to work for them. Taking new powers any other scene, just because you have thaumaturgy is an abuse of the temporary powers rule, I feel. If you want to be able to switch out powers, make that part of your concept and build, don't expect a free meal.
I need a way to balance it. Some of what I read suggests pieces of the power. Each power actually provides several concrete benefits. I f I need to hit someone hard maybe that would be the aspect of the power the ritual provides for a time. If I need to move boulders for a time a different ritual. Much more limited similar costs and it really doesn't matter if you have a the ability to lift a large boulder for a month since when is the next time your going to need that. ( can you imagine Harrys door sign, no parties, no love potions, oh you have a boulder that needs to be moved...right on that)
That's sort of what enchanted items or better yet potions are supposed to do. You can create enchanted items with thaumaturgy rituals stored in them, supposed to act as skill replacement rolls. That can be pretty much any skill and any application you like, as long as you choose them when you make the enchanted item. You could also create an enchanted item that grants you a "supercharged muscles" aspect (or more, if you can put enough power into it), that you can tag on a subsequent fists roll. It will basically give you the same (or a better) effect than strength powers would give you, and it is well within the realms of what is already possible.
If you have something that you'll rarely use, but that you still need, keep some potion slots open. Potions are basically one-use enchanted items, and if you keep potion slots open, you can later declare them with a successful Lore roll.
The reason I am concerned is there are great many nasty's that simply shrug off magic. And unless I want everyone to run around with soul fire or any of the homegrown abilities Ive seen that automatically bypass weaknesses A wizard has to have a way around it with magic as that is his bread and butter. Thus ritual that grant powers.
Well, any wizard can still use mortal gear, especially weapons. Harry does so on a regular basis. And there is nothing wrong with magic NOT being the solution. Don't get me wrong, if you want to buff up on a strength power to defeat a powerful nasty that is immune to magic, that's totally fine. I'm just saying that it is going to be a long way around to gather ingredients and such. And since you are usually not alone in the group, another character might have a better way of dealing with the problem, so it would be his time to shine. I know, if you are playing a wizard, you want to solve everything with magic. Hammer, nail and all that. But that's not the only way to go.
