How often to download data from the API?

Question

API

On master-detail pages:
(For example, in an online store: product catalog and product detail pages)

1. Is it necessary to load data for detail on the master page, if it is possible not to load them? On the one hand, saving traffic, on the other hand, the speed of transitions between pages
   
2. I'm on the master page, went to the detail page, back to the master page. In memory, I have all the data to render the master page, but I can't be sure it's up to date. Load again?
   

If a SPA or a mobile application has a header with the user's initials and a userpic, then is it necessary to pull on each transition between pages GET /whoamiin order to always have up-to-date information?
Are there any other similar tips on the topic? And are these patterns different in the mobile app and spa?