If you know exactly when you are going on vacation, it is great to plan about 6 months or more in advance. That way you can shop for the best deals, and you usually get a much better price for your flights. Last mintue resevations are unpredictable.
This depends on how firm your plans are. If you have specific times you HAVE to go and difficult travel plans, and it's more important to get the specific flight you want than book as soon as you have your plans set. If you are more concerned with saving some money, try waiting for a last minute deal.
For a specific event, as far ahead as you like, but consider travel insurance in the event that something happens, such as illness. Leave extra time to get there for a really important event.
That depends on what time of year you plan to travel. During peak times such as holidays and the summer vacation months buy tickets at least 3 months in advance.
Buy your tickets as soon as you decide where you are going to go. Airlines and hotels sometimes charge very large amounts of money extra on bookings made within a month of a travel date. An exception to this is when you are traveling because a family or friend dies, airlines will give you a special rate if informed.
Purchase tickets as far in advance as possible so you have time to scour the Internet for deals. You can sometimes find cheap fares at the last minute as well. However, I wouldn't recommend this unless you have no other recourse, especially during peak travel times.