[quote=Ichthys]
 I think those are suitable constraints, and I guess to explain the function of the spell, both the spirit and soul of the caster are sent to the raven (so we don't contradict ourselves, regarding the realm's mechanisms). Therefore, the caster, when using the spell, is literally a sack of potatoes, until mind and heart are returned to the caster. With this, I would say the consequences of the spell level out the functions enough that you don't need a long casting time, perhaps just a few incantations and a short ritual, but nothing inconveniently lengthy.If you even wanted to go a step farther, you can have the spell exchange the raven's spirit for the caster's soul and spirit, meaning that while the spell is in effect, the raven has control of the caster's body, although it can't stop the spell because the raven-inside-the-caster still has no soul to do so. This is a bit of an extreme effect, but I can see some major RP potential, in regards to problems, if you take this step. "We forgot to tie up [the character] after they casted the spell. Now, the raven is going crazy and causing some problems." This is just a spontaneous idea I had; don't even consider it as a suggestion, unless you want to. Don't know why that'd even deter you, Tatsua. :p I've known you long enough to know that you're better than that, but you don't have to feel pressured to join. Do whatever you want.
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Just to throw in my own two cents on the ability, remaining too long in the Raven's body could make the transfer permanent. The spirit could be overridden by the animal body, although I imagine the mind could be retained, but if the spirit was overridden by the animal instinct, he could be stuck in its body with his soul trapped. Kinda like animoprhs.