Hello Jan2010bride,
We are getting married in December 2009 and we sent our invitations out the middle of March. So it is about 8 months ahead of time which should be ample time to allow people to book vacation time off work, budget, etc. The rates for November and December 2009 are not available yet though, but we do have the wedding date booked with the hotel. On our invites, we put the exact date of the wedding and put a recommended vacation time for booking holidays.
For example:
Wedding to take place Wednesday, December 2, 2009
Recommended holiday booking time from Sunday, November 29 - Sunday, December 6, 2009.
On our RSVP's, people put their email or phone number if they wanted to go. Once the rates are released, I'm just planning on giving the RSVP cards to the travel agent and have her call each guest and set up the booking. Takes the pressure off yourself and having to deal with money for everybody for deposits.
Also, not everyone may be able to fit in your block grouping and if they atleast know the date, they can arrange to have a mini-vacation to make that.
Hope this helps!