No need to stay on it constantly. Eventually you will become saturated and the benefits will trail off. I usually go 2 on 1 off.
One thing, for me at least, creatine screwed up my blood work I had done for a routine check up from the doctor. It threw my thyroid levels off. We tested my blood 2 more times after that once more to be sure there existed a thyroid problem, the test came back bad again. Then I came off for 2 months and re-took the test and it was fine. It was most likely the creatine, however, it just effected the blood test, I had no real symptoms of any thryoid problems..
Mike