*takes the rat in both hands and drops it in the cage*
It can change at will, but it can only remain in its unnatural form for a couple of hours, maximum. If I could do the same for you, I would estimate you could hold yourself in a human for about half a day.
And it needs to eat a lot before it can change. Obviously it becomes physically a lot bigger and needs more mass. But the process doesn't seem to be very tiring for it.
It's hard to tell what happens exactly, but the energy and bulk of the food seems to be... kind of stored for the next time. I can't tell how or where, though.
Plus it behaves like a mouse, not a rat, and it remembers things it learnt as a mouse, so I believe its 'mind' is completely retained.
Sounds good to me. So it means, I could eat before I change, and then eat afterwards to uphold the change for a little bit longer. You know a cat's stomach is not that big!