1. As much time as I feel I need (hobby dev)
2. For sure I get many ideas especially when outside. But ideas are cheap though and you don't get to test them during a break.
3. Best activity while taking a break is going jogging (and then showering). It's like the exact opposite of sitting at a desk und helps me feel balanced, and be productive, concentrated and focused. I think breaks certainly need to be without any form of electronics or they aren't good breaks. Best is going outside. At least open a window and let fresh air in. Closing your eyes for 15-20 min ("power nap") is also excellent to combat fatigue.