For most local weddings and events, set your RSVP deadline about 3–4 weeks before the date. That gives you a week or two to chase late replies before your caterer or venue needs a final headcount.
For destination weddings, ask for responses 6–8 weeks ahead. Guests need extra lead time to book flights and accommodation, so an earlier deadline keeps everyone organized.
The reliable way to set the date is to work backward from your caterer's final-count deadline. Take that date, add a buffer week or two to follow up with non-responders, and that buffer date becomes your RSVP deadline.