When to Choose a Photographer
The short answer to "when to choose a photographer" is as soon as possible, if photos are important to you. Popular dates can be booked anywhere from 6 months to a year in advance.
But don't panic if your wedding is less than 6 months out, because chances are you still have plenty of time, especially if you're not trying to find a photographer for the most popular time of the year in your location. (Here in Arizona the most popular times are March/April & October, elsewhere it may be the summer months.)
The best time to choose a photographer is as soon as you are able to after your wedding date, officiant, and general location have been set. If you live in a large metro area, you probably don't need to have the exact ceremony & reception site set before you can begin looking, but if you live in a rural area or a planning a destination wedding you'll probably want to wait until after you have the exact location. The closer to your wedding date it gets, the fewer choices you will have, so if photos are important to you, begin looking early.
Most photographers will put a 24 hour temporary hold on a date if you request it, but beyond that the date won't be held unless you've paid a retainer.