I usually baseline thaumaturgy duration at "one scene". If you shift into a bird to fly up a sky scraper, the spell lasts until you are up there. If you turn into a bear to fight, it lasts until the fight is over. I equate that roughly with the 15 minute step on the ladder per default.
When you ask "what kind of spells [...] can be applied?", do you mean how to do a shapeshifting spell? If yes, I would do the usual "5 shifts takeout". It uses the fact that you can do basically anything to a target once you take it out, and you'll need a maximum of 5 shifts to take yourself out, since you won't take consequences if you want to be taken out. And since you used magic, you can now justify having turned yourself into a rabbit.
If you want to have powers to go with that, you can go with the temporary powers rule (YS92). Either spend Fate points to buy the powers or you can gather up materials and use the tag on the aspects as payment for the powers. Though I would be careful as to what to allow for this. Just a bunch of quick discipline maneuvers won't be enough to tag for a rabbit power, but a freshly skinned rabbit might.