A word about time-boxes before we start: all Scrum events are time-boxed. For those that are formally part of the Scrum framework, their time-box recommendations are in the Scrum Guide (https://www.scrumguides.org).
All other activities or events that we introduce, such as Product Backlog refinement, should also be time-boxed. For these time-boxes, the facilitator of the event should estimate how long they think they need. They declare the time-box at the beginning of the event as part of the setup.
If you hit the time-box and you still have work to do, you can ask the team what they would like to do. Should they carry on, and if so, for how long? Should they reconvene at another time, perhaps?
Remember, meeting fatigue is a real thing, and people may have other commitments. Finding out if they can commit more time to extending the time-box is only polite...