Because wildshaping as a Druid is fun and many of its possible polyforms are situational, the cooldown for entering a new form should be short in most cases, but there should still be a penalty for using a form just as a source of bonus HP. Therefore, it should follow these rules:

• If your wildshape polyform times out and you return to natural form, there is no cooldown at all and you can immediately wildshape again.
• If you manually cancel your wildshape, there is a short cooldown on the order of a couple hundred turns.
• If you are killed in your wildshape, there is a long cooldown on the order of 1000 turns.

You might also be able to ignore the cooldown and force a shape change, at the cost of magic power and/or nutrition proportional to either the remaining cooldown amount or the strength of the form you want to take.