It is fashionable to quickly send a small thank you card (if not a message) but above all you have to return the invitation.
Ha good, Duchess, but how soon?
If you did not know your hosts well then you have 1 month to issue the return invitation. And if they are your regular friends then it goes to 2 months. But don't get me wrong throwing doesn't mean receiving… we can play a little on the deadlines!