Depends on how long the authorised seller takes to register them...
Ours are always in DEP before they get onsite, but that would usually be at least several days after purchase.
No idea for a walk in and pickup, but I am guessing the biggest delay is still going to rest with the vendor.
I do know if you have automatic JSS enrollment turned off it's something like 20 - 30 minutes after enabling a device that it will show up in the JSS, but that is a different process from the vendor registration which has to happen first.