I'd like to present some data, using which I'd like to understand how much time should be reserved for backlog refinement. Don't get me wrong, I understand that refinement is not necessarily an event or a meeting, I understand that it can be an ongoing effort (if needed) or it can happen in whatever manner that best fits the situation of the team.
Now, per the Scrum guide
Refinement usually consumes no more than 10% of the capacity of the Development Team
So, Let's assume the following:
Team 1
Team Size: 9 Developers