how does an unconformity indicate a long time has passed?

Answer 1

Well unconformities represent missing time, also unless you have a complete rock record elsewhere to compare it with it is difficult to determine how much time is missing from an unconformity alone.

Answer 2

Not necessarily a long time, but that there is missing time, they can cut through different layers, and the type of unconformity, especially angular, can sometimes give clues to the amount of time passed. Usually you need another area of exposure to compare***