Straight from the AS5 website. Basically 200Hrs of burn in time and several "thermal cycles" are needed. So basically keep it on for however long you need to then power it off to let the AS5 cool down (over night I guess) then power up again and run it for a day then power down. Do this a couple times and the AS5 should cure and transfer heat more efficiently after the particles cure and start to do their magic.
Due to the unique shape and sizes of the particles in Arctic Silver 5's conductive matrix, it will take a up to 200 hours and several thermal cycles to achieve maximum particle to particle thermal conduction and for the heatsink to CPU interface to reach maximum conductivity. (This period will be longer in a system without a fan on the heatsink or with a low speed fan on the heatsink.) On systems measuring actual internal core temperatures via the CPU's internal diode, the measured temperature will often drop 2C to 5C over this "break-in" period. This break-in will occur during the normal use of the computer as long as the computer is turned off from time to time and the interface is allowed to cool to room temperature. Once the break-in is complete, the computer can be left on if desired.