From what i have read fat cells have a pretty long half life.
Obesity is increasing in an epidemic manner in most countries and constitutes a public health problem by enhancing the risk for cardiovascular disease and metabolic disorders such as type 2 diabetes. Owing to the increase in obesity, life expectancy may start to decrease in developed countries...
pubmed.ncbi.nlm.nih.gov
And they don't really ever go away.
Researchers have found that the number of fat cells in your body is set during adolescence and remains constant through adulthood, regardless of whether you gain or lose weight. The findings may help to explain why it can be so hard for some people to drop pounds and keep them off.
www.nih.gov
It would be nice if this really isn't the case.