[necovek]

As soon as you've got something releasable.

Though, you should break work up in a way to get to something releasable asap.

The latter is where I think it's more art than science still, or at least I can't come up with a good written process on how you do it (other than the constant "where is the value in this" and "what's the smallest thing we can build" questioning), but I can always do it!