I have a pair of wheels with Suntour sealed bearings from, probably, the early 80s. I pulled the seals and found they were excellent with good grease. I smushed some new grease in just cuz I was there but they are doing great and sound-n-feel very smooth.

I now have an old wheel from, I'm told, a Viscount Lambert with sealed hubs. Someone may have an idea of how old these might be. I've pulled those seals and the grease is fine and the bearing are clean and smooth. I'm guessing that a sealed bearing can last a very long time.

However, as with anything, quality of a sealed bearing can vary all over the map. There's cheap ones that give up the ghost too soon and real good ones that last a very long time. Buyer beware.