I think every bride has slightly different expectations for their RSVPs. And of course, guests usually have completely different expectations
Our wedding is April 2011. I mailed out STDs in February 2010 because I knew that a lot of our family and friends would need a good year or so to save up the money if they wanted to come. We then mailed out our official invites with all the travel/booking info in June 2010. I asked for a RSVP deadline of Feb. 1, 2011. To me, two and a half months is more than enough time get a final count and plan party favours, OOT bags, etc. but every bride is different. I didn't want it to be too far in advance as some people try to book last minute hoping to get a good deal (although if they get a better deal than our group rate, I'll be shocked!). However, I made it VERY clear (in a diplomatic way, lol) that it IS a wedding and requires planning so if you haven't booked by Feb. 1, 2011 then you'll be assumed to not be coming. I also asked people to RSVP only when they are 100% sure whether or not they are coming...it's too confusing to get people to RSVP "yes" when they haven't even booked yet and then may eventually back out (I was pretty clear about that too in my invites).
I think it's best to be clear with guests what you expect as most people have never attended a destination wedding before...in my experience, everyone really appreciated the info and "direction" provided, haha