WoW game time in epic pack

Can you choose when to use the game time? Or it activates from the moment you purchase the collection.

My understanding is that game time blocks from bundles like that is applied immediately to the account. So, it would mean 30 days access to WoW Classic, and everything else in Retail WoW except Shadowlands (most recent expansion).

If you already have a Subscription (renewing game time charge) then it won’t renew until after the Epic 30 days runs out.

Well, I don’t have a subscription going on so it won’t add up to it. I wanted the pack mostly for the Diablo III transmog set, so if I buy it now without planning to play WoW right away, I’ll waste the game time?

Correct. Unfortunately that is how it works.