Depends on what kind of party you're having. I had a party that was like a mini-wedding, so I sent them out about 6 weeks ahead and made the RSVP date 2 weeks before the party. I needed enough notice to order the rentals and to give to the reception hall
Re: Baptism Invitations...
I agree with 3 weeks. I did the same thing, but I had told my family the date and time earlier so they at least had notice.