Mine is 4 days before, only because my hostess is cooking it (with my help of course). If you are catering they will give you a date that a head count is needed by so about 2-3 days before that.
I think my host set an RSVP date of 2 weeks prior to the shower. It really depends on where it is being held and how far in advance they need a head count.
Mine was 10 days before the shower, as that was what the venue requested. There were several changes after that, but not enough to impact the number of tables or room size we needed.
Re: How far before the shower should the RSVP deadline be
~Benjamin Franklin
DS dx with celiac disease 5/28/10