How does the 24 hour grace period for Virtual Datacenter Subscription work in Satellite 6.x?

Resolution

When a VM registers it may consume one of the temporary subscriptions provided by a VDC subscription (or other flavors of subscriptions that provide "guests").
This temporary subscription will only be available for 24 hours after the content host registers.

You should see this Temporary subscription on any registered content host that is running on the hypervisor in the subscriptions tab.
A screenshot is attached at the bottom of this page for your convenience.

Note: It is not necessary for a subscription to be attached to a hypervisor for the temporary subscriptions to be shown as available.

The temporary subscriptions are meant to be there for auto-attach and not to be added to activation keys.

What if I want to provision VM's with an activation key?

There is a bugzilla RFE open to improve on this: Allow activation keys to enable product repos regardless of whether there is a subscription attached or not

There are however two possible workarounds that can be used:

Workaround1:

The solution would be to register using an activation key without specifying the VDC subscription in it, relying on the auto-attach to assign the temporary subscription. 
Then in a second step, use subscription-manager to enable the repos desired.
All of this can happen before the virt-who mapping has occurred.

Workaround2:

1) Create two activation keys  Key "A" and Key "B", assign them both to the same lifecycle environment and content view
2) Assign no subscriptions to Key A
3) Assign a VDC subscription or some other subscription to Key B that provides content.  This subscription does not even need to have any free entitlements available
4) Adjust the enabled content on Key B
5) Register a client with  --activationkeys=A,B

The guest will use Key A to auto heal itself grabbing the guest subscription and then use Key B to grab the list of enabled repos.

Workaround3:

Update the virt-who interval to 5 minutes for faster mapping.
Edit /etc/sysconfig/virt-who
Edit the following line:
VIRTWHO_INTERVAL=300

Note: We are trying to discourage decreasing the virt-who interval to be so short as it takes a higher load on the Satellite and VM servers and sometimes for large infrastructures the time for the mapping takes longer then the time for the newly provisioned VM to reach the registration step which means this workaround would not work

For more KB articles/solutions related to Virt-who and Virtual Datacenter (VDC) Subscriptions Issues, please refer to the Consolidated Troubleshooting Article for Virt-who and Virtual Datacenter (VDC) Subscriptions Issues