Sandbox Clone with a Different Version from Production Takes a Long Time to Complete

This refresh may be taking longer than usual for any a number of normal conditions that factor into the duration of a sandbox copy or refresh, including the number of customizations, data size, numbers of objects and configuration choices (for full copies), as well as server load.

Sandbox refreshes might complete in hours, days, or even more than a week. This is due to this type of sandbox being on a special queue. With that said, it is also likely that your requested refresh may not start immediately after you initiated it. 

For these reasons there is no Service Level of Agreement (SLA)  for Sandbox copies and a time until completion cannot be quoted. 

Salesforce Support does not have the ability to speed up or prioritize clones due to special sandbox queue restrictions. Copies must remain in the same queue and processed naturally on a first in first out basis.

As a workaround, if you need a sandbox as soon as possible, you may opt to refresh a brand new regular sandbox as non-clone sandbox has more queues to use and can likely complete faster.