As far as we know, episodes of this illness come and go in their own peculiar fashion. It is not unusual for the visible symptoms to wane for many years, only to recur when the individual least expects. Furthermore, the actual symptoms we call "mania" and "depression" represent wild exaggerations of a more basic problem---in this genetically transmitted disorder, people do not experience innate affect in any way like the rest of the population. Most of us know exactly what range of phenomena have to have just happened in order for us to experience any emotion; that's how we can organize our emotional lives into scripts that match those of others. But in Bipolar Affective Illness, sequences of stimulus increase, stimulus decrease, and stimulus level (the normal triggers for innate affect) do not trigger reliable felt experiences. What brings on the normal range of interest through excitement at one time may bring on a wildly amplified version of that affect at one time, or a barely felt and terribly attenuated version at another time.
I've worked closely with a wide variety of people who have a wide range of involvement with this illness, and am willing to state with all the confidence I can express that this variability in response to normal stimuli causes great difficulty in the establishment and maintenance of normal relationships. If we know our own experiences by the affects associated with them, then we know (or think we know) the experiences of others on that same basis. It is difficult enough to know the inner life of another person, and this experience of the likeness of one's affective life to the affective life of another person is a vital part of normal relatedness. To the extent that another person truly does experience affect differently from us, we start a relationship under great handicap.
In answer to the basic question you posed, I do not think any person ever is entirely free of this illness. Even though one may be free of the extreme manifestations of it that earn the "diagnostic" label, (in my opinion) it always makes people different from others to a significant degree. Finally, no matter how long one has been free of an episode,it remains possible that a further one will occur.