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.
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+1 - It's also important to only use a tiny bit of paste for this reason, as it should only be filling the tiny pits in the surfaces and not stopping them from making as much contact with eachother as possible. Heavy overclockers often do not use any thermal paste at all, after 'lapping' the surfaces. http://www.google.co.uk/webhp?hl=en#hl=en&rlz=1R2GGLL_en-GBGB392&q=heatsink+lapping&aq=f&aqi=&aql=&oq=&gs_rfai=&fp=e37afde761aa092a
– paradroid – 2010-11-17T05:38:44.470Follow the vendor's instructions as to how much paste you use (if you ever need to). It depends a lot on it's constituents and how much pressure/heat it requires to make it spread out. Using a "tiny" blob of something like Noctua NT-H1 will result in a cooked chip. I normally settle for an overnight burn-in as it checks the chip is running cool and allows the paste to spread under a constant load. – Tog – 2010-11-17T08:13:15.090