You should start sending out CDs and Press Kits a minimum 6 Months before the first date you want to book for your run. So if you want to start a tour on August 1st, you better have all of your clubs targeted and your packages sent by February 1st. You can obviously put a tour together in less time and many have. However, if you want to have a smooth trip with as little stress as possible, good routing and with only the best opportunities, then you want to plan as far in advance as possible.
It is also a good idea to choose two sets of dates a few months apart, that you and your band members agree are possible to start and end a tour. For example, August 1st – September 1st and November 1st – December 1st. Now if you get someone interested in booking your band that does not have the dates you want available, you will at least have a second option of dates that might be available. You should keep track of every offer that you receive and find out how long you have until you need to confirm the offer. As the tour gets closer you will be able to make a better informed decision about your routing based on all of the offers you have.