When they touch your ears ?
Every given period of time, like every 2 months ?
When it just doesn't look right anymore ?
I usually cut mine every 8 weeks, and that seems to be good for me. Then I find it clean but too short for the first 3 or 4 weeks, then I find it perfect at the 4th week, then a little too long once it gets to 8 weeks long.....
How about you, what makes you decide that it's time to get your hair cut ?