The frequency depends on what's going on. If I am doing well my GI doc may order one every once or twice a year along with or in leiu of a C-reactive protein test. If I actively flaring he almost always orders one.
Mine has always come back normal, below 20.
It's often considered an inconsistent test and a lot of docs don't put their faith in it although it one of the only tests that checks for generalized inflammation.
Hope this helps.
MyUC