Just got done changing mine out, and I do it every 6 months. 6-8 months is what I think is normally recommended. Every time I do it, I'm amazed at the change in brightness and color between the old and new. They degrade so gradually over time, I just never notice it. I can't imagine the difference if I let them go for a year.
Also... time will vary depending on how long you have them on to start with. I run mine 10 hours/day.