If both your parents have bipolar disorder, you have a 50-75% chance of inheriting the disorder; if one of your parents has it, you have a 15-30% chance of inheriting the disorder.
There are many things that influence the development of a mental health difficulty. With regards to bipolar disorder, which falls under a broader diagnostic class of psychosis, there seem to be quite a few things that can influence whether someone will develop the condition. Remember that bipolar can develop due to a number of different reasons, not just through genetic inheritance.
Briefly, bipolar, previously known as manic depression, is when someone shifts from feeling very depressed, lethargic and fatigued, to feeling high, manic and acting very spontaneously.
There is evidence that certain types of psychosis appear to run in families, however scientists can not agree to the reason for this. Some believe that there is a gene that means the condition could be inherited. However, there are also many psychological and social factors which contribute to its development. Just because someone's mother or father has this problem, does not mean that their children will get it too.
As a clinical psychologist myself I have to say that I tend to think of the symptoms of bipolar as being a way for someone to cope. When someone has something traumatic happen to them, be it a one off event, a series of small events or difficulties in upbringing, it can often lead to that person not knowing how to cope or deal with the situation and memories. As humans we are great at being able to cut ourselves off mentally, and avoid having to deal with thoughts that might be too difficult to cope with. Therefore someone would be more likely to develop bipolar disorder if they did not have the coping resources needed to deal with a trauma or significant life event. Being high means your brain is running too fast to process the trauma, and being depressed means your brain is running too slow for it to be able to concentrate or focus on it.
Therefore the risk of developing bipolar is not purely due to it being hereditary, so it is impossible to put a true figure on it.