There are LOTS of ways to do it.

For me, I decide on an amount of weight and a length of time - whichever comes first, that's when I stop bulking. So, last fall I dedided to bulk until EITHER Jan 1 OR I gained 10 pounds. I had gained just over 8 pounds by January 1, so I started cutting then. I continued to try for several months, dropped a few pounds, then decided my body was building well, so I went on another mini-bulk. Basically, for me, I don't like my weight to oscillate too much because I like my clothes to fit. If everything in my wardrobe was made of spandex, I'd set size goals - like, stop bulking when my biceps hit 13" or something.

Right now, I'm doing one week on and one week off. You can even vary the programme day by day - carbs on lifting days, low carb lower calorie on cardio days. It just depends on how your body responds and on your particular goals.