Changing forms wasn't intended to take that long. He wouldn't be able to slip between forms like he's performing different moves in a combo of a fighting game, but it would only take a few seconds. The Werewolf and Vampire weaknesses in particular would be able to halt his transformations though. The legendary beast forms would probably take longer, though even then still only a dozen seconds or so.

First stage caterpillar is an actual mundane caterpillar, second stage is about the size of a large cat or medium-sized dog. I figure I'd skip the part where he goes around munching on leaves.

Also, I've realized I've neglected to clarify that his evolutions are supposed to be more like digimon rather than pokemon; he can go back to previous forms, though there usually isn't much incentive to do so besides unlocking other evolution paths