One option might be to use the complexities for Conjuring items as a baseline, with the assumption that the items revert to a reasonable analogy of their original form after about a scene. By this I mean that the partly digested steak would revert to a partly digested burger, and so on. You could increase the duration by increasing the difficulty. Unlike Conjured items, Transformation cannot simply be dispelled.
I'm not sure I really like this solution, since it seems as though a Transformation ought to be permanent, but it seems reasonably balanced.
A possibly better solution would be to use the "Solve improbable or impossible problems" clause of Thaumaturgy. So basically, you ask yourself "if it was even possible to re-chef a burger into a steak, how would it work?" I'd figure out what skill level you'd need to make the item (which might also be determined by your desired quality, but probably defaults to around 2 for baseline quality), then figure out how long it would take for a craftsman to do the work. Then reduce the ritual time to a reasonable amount by adding complexity (see the time chart).
1) Transform a fast food burger into a steak dinner? It doesn't take much of a cook to prepare a steak, though it's harder without good meat. Perhaps a base complexity of 2-3 with a base time of a half hour. The base complexity (before adding complexity to reduce time) would indicate the quality of the steak.
2) Transform a knife into a sword? Similar to above, with base complexity determining quality of the blade. Base time might be several hours or more, reduced by adding complexity.
3) Transform a pen into a knife? Same as for the sword.
4) Transform a wall with a door into a solid wall (with no door)? Are we talking drywall, as typically used for house interior walls? Probably complexity 2 and an hour or two. A brick wall? Similar complexity (maybe a point higher), but with an afternoon base time. Solid stone? How long would it take to chisel the shape, days perhaps?
5) Transform a house that burned down to its original condition? Probably a base complexity of 2-3, with a base time in the weeks or months.
So as an example, rebuilding the house with 'fair' worksmanship by use of an hour-long ritual would be 2 (skill) + 8 (reduce from "months" to "hour") = 10. Transforming a pen into a 'good' dagger with a ritual of only a few minutes might be 3 (skill) + 4 (reduce "hours" to "minutes") = 7.
Does this look about right?