Strictly, I think you may get it no later than Aug, 2007.

And if your previous company revoke your LC or I-140, you cannot stay later than Aug, 07. But I saw lots of people all agree than you may use all H1-B time which has been already extended, as long as your previous company does not revoke LC/I-140.

This kind of transfer is not guarantted to when. So, if I were you, I will wait until the H1-B transfer is approved, then you will know how far it can go.