How long ?

How long does it take you to start conversation over the email using the recipients first name ?Is it after a single email, after two ?

I have observed that when starting a conversation with some one that I don't know and the first interaction is over the email; I start out in a formal way saying Dr. family name. But after a few email exchanges I find it difficult to keep conversing in a formal tone and I get to the point and start using first names.

I don't know if that is a good thing and how do people perceive it.