Lifestyle changes definately help prolong the life of the grafts but there are still limits. Veins are not really made for the high pressure of arteries and usually close after 5-10 years with an average of 7 years. ARterial grafts (e.g. mammary artery) on the other hand can last a long time. I have seen at least one person with a 20+ year old artery graft that was still wide open.
How long do the bypasses last?
I think I've heard more poor information about this than about any other topic in cardiology. Bypass grafts can last for really long periods of time. I have seen many instances where the graft looks almost "brand new" on angiograms done 15-20 years later.
Bypass grafts do close however. About 50% of the grafts placed are closed in 10-12 years ( that means that if four bypasses were placed in a patient, then 2 would be expected to be closed in 12 years). The ones which do close most often are those that supply smallest vessels, or those which supply the heart muscle with the least amount of need for flow. This makes sense to a great degree - veins are soft structures which can collapse, and if the flow is slow, they may do this early. The patient is generally not aware of this when it happens either since there may be enough flow from other vessels to "make up" for the decreased flow through the bypass.
Again, it is important to take good care of yourself and your bypass grafts. You and they will last longer that way! Recent information regarding the use of cholesterol lowering agents stress this: patients who were given cholesterol lowering agents lived longer with more open grafts.