Welcome to the site. Looks as if I responded a year ago, so I'll reply again. Check out http://www.medicinenet.com/script/ma...iclekey=157905. It doesn't answer your question about how long, but there's useful information. Mine was found by a doctor who had the good sense to order a CT scan. I was followed up with additional CT scans and a PET scan.

Do you suspect a thymoma?

Jim