How soon in advance of a large gathering should invitations be sent out?

Let's assume large = 100 ppl.
If it's a wedding = 8 weeks / RSVP due back 3 weeks prior
If it's a family reunion = 8 weeks / RSVP due back 2 weeks prior
If it's a yard bbq = 3 weeks / RSVP due back 1 week prior

at least 2 weeks in advance is a good practice..=)

At least one month. People often make their plans this far in advance.

Two weeks beforehand. One week to confirm who's attending the gathering, the other week to think what food or anything should be prepared.