Now, ITV haven't given any indication of specific schedule but it does seem likely that Philip's episode (Curtain) will air before the end of this year or very early next year. If the episodes are to air weekly then Curtain should air around the 13th November, if monthly then it will be January 2013.
Hopefully it won't be any longer than that - David Suchet has tweeted us to say that he thinks it will be before the end of the year!
EDIT (15/10/2013): The 2nd of the four has been confirmed for the week after the first - suggesting Philip's ep will air on 13th November. We'll let you know when this is confirmed.