Product Priotization has to be a continuous exercise. Teams continually reprioritize items in their backlog to ensure they are spending time on the right things.
A few surveys (ex. one conducted by www.productplan.com) shows that more than 40% of the Product Owners, do a weekly prioritization.
Having said that, there is not exact frequency set as silver bullet. Every team arrives at their sweet spot by balancing their constraints and opportunities.