The general rule for planning a multi-day conference is to have your dates and location set at least one year in advance. Annual conferences typically announce the details for the following year during their event. This works well because it gets everyone to set their calendars in advance, and if you launch an early-bird program at the same time, you should walk away with a nice deposit towards next year's event.
If you are planning a conference for the first time then you should probably begin 18 months ahead. This gives you time to navigate the learning curve and have a better chance at landing your first choice of venues. Keep in mind that competing events can steal from your registration pool, so browse through the calendars of organizations and associations related to your industry and eliminate conflicting dates.
The timetable will of course need to be extended for those planning large conferences with thousands of people. In these cases you will likely work with a visitor's bureau to coordinate all of your hotel and venue accommodations. This process takes at least two years and potentially longer if you expect the city to provide public space and security during the event.
Conference planning is a big task and certainly not one you want to tackle by yourself. I recently put together a collection of advice and articles that will help you get started on the right foot. You can read it here: Step-by-Step Conference Planning Guide