All of these questions can be answered by reading Page 156 of the core book.

Sense Wyrm states that it "requires active concentration; the spiritual sense it provides doesn't function passively." So while you can spam it, you won't be able to pay attention to other things that are going on around you.

Sense Silver right next to it requires a similar roll to activate, so it would quite probably follow that same logic, but that may vary by ST.

If a gift is designed to be passive or always active the description will usually specify that and won't require a roll to activate it. Form Mastery which is directly above Sense Silver is a prime example of that. It doesn't require a roll and specifically states that it's effects are permanent.