Sealed, in the original bottle? For an obnoxiously long time.
I highly doubt you'll have any issues with the paint. Give it the good typical shaking required of paint that's settled with time, and you'll see no difference in quality.
The only factor that usually interferes with paints that are still sealed is excessive exposure to heat/cold.
"It is curious that physical courage should be so common in the world and moral courage so rare."