First for the 200 hours statement: It depends on type of thermal paste. There are pastes like Arctic Silver which need time to settle and there are thermal pastes like generic silicon based or Arctic Cooling MX-3 which do not need to settle.
Next point is about the way paste settles. It's not just the time itself that's needed. Volume of thermal paste changes with temperature. As it changes, paste starts to move into surfaces and fills irregularities. As it cools, it starts to move back to its starting position. That is why it is important to turn computer on and have it running for a while and then turn it off and have it cool for a time while the paste is settling.