I take it you are English eh?
There is no minimum wait time. Assuming you properly installed the HSF after applying the TIM, the TIM is fully spread out and functional before you even press that power button.
Arctic Silver 5 takes time to gain maximum cooling effectiveness, per the manufacturer, but you must use your rig in order for this settling period to be carried out. (it relies on the thermal cycling)
Bit all other TIMs are ready to go once applied, and once your HSF is applied your computer is ready to go.