Not much, a very thin layer, I use around 1/2 pea size, spread with clean fingers. You don't want any dirt to cause gaps. The purpose of the thermal grease is to fill in imperfections and prevent air gaps between the cpu die and HSF which would insulate and lower the heat transfer rate (actually the heat transfer rate stays the same except a higher temperture would have to be had to drive the same amount of heat transfer rate). Too much would cause an unnecessary layer of grease which doesn't conduct heat as good as the two materials joined together.