Count on at least three months for a small to medium-sized activation, and six months for a large multi-location campaign.
The lead times aren't determined by creative work (that can go faster), but by permits, venue agreements and production delivery times.
A municipal permit for public space usually takes 6 to 8 weeks to process. Festival sites and exhibition stands are often booked half a year to a year in advance. Bespoke-built settings need 4 to 8 weeks of build time.
Anyone who plans less than three months out almost always ends up with compromises in venue or delivery.