I understand that the Learning Budget allows us to systematically spend budget to learn about unexplored keywords. I have set my learning budget to 10%. When can I reduce my learning budget?
Learning budget helps us to explore the potential of bid units (keywords/ads) that do not have impressions, clicks. If a significant number of your bid units have clicks, then you can decrease your learning budget. The way to check that is by looking at Coverage (in Portfolio Settings).
Another time you may want to decrease your learning budget: when you cannot increase the budget and see a decline in the Portfolio's performance. A large learning budget will divert a big chunk of the budget towards bid units that may or may not perform well. In this case you can (at least temporarily) lower your learning budget.