@WreckingBerg From a lot of posts I've seen on here and Reddit, Stadia support are saying it may take 3-5 business days and to contact them if you still haven't received the code by the end of the week.
@WreckingBerg They're still in the process of rolling out the activation codes. You can check Google's response below:
https://community.stadia.com/t5/Stadia-General/Response-from-Google-with-regards-to-delayed-activati...
Also you can check the 'Solution' on the pinned post on this board:
https://community.stadia.com/t5/Stadia-General/Ordered-6th-of-june-yet-to-recieve-a-code/td-p/255