It's a good rule of thumb when changing anything with your system to flush every 4-5 weeks. As you become more experienced, you will find you have to flush very little. I probably flush a little different than most people, because I usually run low nutes to begin with. This makes it so I pretty much flush only when I see a problem, or before harvest. And my before harvest flush is more of a tapering than an "oh shit let's try to slam some clearex down this slut our last two days of flower."
@ Forver: You can just flush that day or for a few days, depending on the medium you are using. Then resume feeding at 1/4 strength the next time you water the plant, which should be a few days +. Then taper your way back up to what you normally use nutrient wise.
TDS and pH meters that are calibrated frequently are a good thing to have around to keep some of the guess work out of things.