For paste such as Arctic Silver, it's generally recommended to wait 72 hours for the CPU die to 'fill in'
Remember folks: NEVER bother using a thermal pad and paste simultaneously, and if you got one of those crappy stock heatsinks, it's really better to just scrape off the 'chewing gum' pad, clean it off, and re-apply with some Arctic Silver - a large tube of the stuff doesn't cost very much, and many have reported better cooling from the stock cooler this way.