[WannaBePearShaped]

Hi Charlie ,no you don't have to let it sit for 3 weeks after that;heating it up will already get the benefits from the herb.If you want,you can let it sit if you want for a couple weeks with more powder or extract in it to increase potency.I'm just going to heat mine for 2-3 hours in a double boiler because I'm impatient.
Here's a video and link for more info:
https://www.youtube.com/watch?v=nrbowas5z6g
http://whisperingearth.co.uk/2010/04/26/potions-group-making-herb-infused-oils/
http://www.anniesremedy.com/chart_remedy_herbaloil.php

Hope this helps you