I had a similar situation with British Airways flights when I booked up around 18 months ago. I wanted to fly from London to Orlando in January 2009 and then return home in January 2010. At the time of booking, the latest return date available to me was in early December 2009. The airline advised me to book the latest return date and then to change it at a later date, when my preferred return date became available. The change cost £70, which wasn't too bad, but do bear in mind that not all airlines will let you change your ticket.
I don't think a greater amount of time between departure and return dates has any effect on price. I paid £325 for my flight at the time and the same price could be had if I would have returned a week or two later.