Whiny transmission: A bad thing?

[DavieGravy]

Upon picking my car up from getting that horribly noisy bearing fixed, the mechanic brought to my attention that my transmission is "whining a bit," which is a sign that it's becoming worn. Is it possible that he's mistaken and it could be something else? I have noticed a slight wine that's barely audible, but nothing bad enough to warrant concern. So how bad of a sign is this? Can anyone give me a ballpark estimate as to how much longer it MIGHT last. I do mostly highway driving, but have kind of a led foot. There's no RPM flare yet.
Thanks in advance for any input.