If you've been, you'll know what I mean. It wafts up from the metro and consists of: blocked drains 30 per cent. something like burnt paper, about 20 per cent.
Guests should have at least four weeks to consider the invitation before the RSVP-by date, which, in turn, is usually one to two weeks before the wedding (ask your caterers when they need a final count).
You'll also want people to RSVP so you have a head count for the caterer. If, on the other hand, your rehearsal dinner will be fairly low-key or small (think: a party at a restaurant or an intimate gathering at your future in-laws' home) then you don't need to be as official with your invitations.