If the trip is already planned and you’re just buying into a pre-existing travel product such as a cruise or major tour operator’s trip, nine months is usually adequate. However, if the trip needs to be designed and then booked from scratch (all the hotels, flights, motorcoaches, guides, etc.), then you should allow at least a year’s lead time.
A year would give you three months to work out the day-by-day itinerary, book the reservations, cost it and publish your promotional materials. Then you’d have six months to market and sell it. You’d wind up with the last 90 days to finalize everything: make final payments to your suppliers, submit rooming lists, prepare and mail out final documents to each traveler and brief your tour leader.
Obviously, one-day or short trips near home can be done easily within a close-in time frame. International trips usually require the longer lead time. Some trips wherein you wish to include major global events such as Mardi Gras, an eclipse, New Year’s Eve, etc., may require booking several years ahead if you are to guarantee tickets as part of your package.