Don't know John that's what I'm trying to find out.  I was told it is 90 days by the London Embassy this morning.  One of the gals here who has already gone through the procedure with her spouse tells me differently.  I will know tomorrow and will post the reponse that I receive from the embassy.  Perhaps there are different timeframes for different visa  :-/
Well here's the response from the embassy John.  And as for my nifty friend Andy - shame, shame on you!!!!  

Once an immigrant visa is issued you must travel to the U.S. to make your first entry within 6 months of the date of issuance. 
Thank you for your email correspondence. 
Consular Information Unit 
U.S. Embassy London 
CONS/CIU/GMS 
This informal method of responding enables us to respond to you within a shorter time. No record is being made of this correspondence. If you need to email us again, please return this email.
THANKS LEAH!!!  ***for everything***  You are an angel to me especially in the early and some of darkest days of living here.  You always knew when I needed to chat and always took the time out to send me a PM to see how I was doing..  You were the first friend (virtual or not virtual, doesn't matter.. a friend is a friend) I actually made here in London when I think about it (and it took the UKY Bar-B-Q to actually meet)   

  Know that your friendship will always mean a lot and stay in my heart when I look back at my journey here in the UK and say yeah, I remember when I lived in London and I remember Leah  

 Cheers...