There is also a possibility of bypass vessels failing at some stage, but nobody can tell you when and if that is likely to occur. For example my bypass lasted just 3 months and I know people who have passed 15 and 20 years.
Kenkeith is spot on with regards to your risk in forming more blockages, the only way around this is severe life style changes. One of the important risk factors now on the list in the UK is stress and in Cardiac Rehab they strongly emphasise on making big changes to avoid this, even if it includes giving up your current employment.
Risk remains the same as before the surgery. To reduce your risk is to follow the recommended protocol: healthy heart diet, exercise, reduce your cholesterol, moderate lifestyle, don't smoke, etc.