I think it largely would depend on getting the proper treatment and sticking with it (which can be very difficult). It also depends on how severe the case is.
If you have questions about bipolar disorder, why not ask your fiance if you can speak with his psychiatrist about it? Write down a list of questions and take them to the meeting. Your fiance can be there or not, depending on what you two and his doctor decides. If your fiance isn't there, however, his doctor won't be able to give you specifics about your fiance's case (without his permission), but you can ask general and hypothetical questions.
Also, you may want to look into a support group for people with family members and friends with BP. They will understand the ups and downs. It may keep you from being overwhelmed. It is really great that your fiance found a patient person who loves him.