Difficult to answer this question because it varies from person to person and and depends on the age. I think it's safe to say that >80% of people are symptom free for at least 5 years after the CAGB. How long the new blood vessels will last for is very variable.
Many people find their CABG lasts longer than 5 years, sometimes up to 10-15 years. To a certain extent it depends on whether or not one makes lifestyle changes (dietary changes and exercise) after the CABG. If not, and one goes back to their unhealthy lifestyle as before, the CABG won't last as long.
If you are having chest pain
again, see your GP who will refer you back to the cardiologist and he'll carry out some assessments. If your graft was done in 2000, it's been 9 years. So it could well be the case. How old are you again?