In B2B it should be done every 6 months or whenever there are big changes in your target audience.
For example - If you’re adding a new feature/product in your catalogue, you need to update your buyer persona aligned to the new feature or product you’ve created.
If you’re a Business SMS Solution and you wish to add WhatsApp as a platform as well, then you need to research on the regions which use WA frequenty and the businesses which function in that region.