Do it for however long you feel like it biologic; however, be mindful of that. I would said doing it for 45 - 60 seconds a day would be good for most people. After all, it is harder than a plank if you do it the right way
I would say definitely. It is a core strength exercise. For average people I would most likely recomend 45 - 60 seconds a day, but what ever you feel confortable with will work also. Don't over do it though, the blood will begin to flow to your head if you do it for over a few minutes. That wouldn't be good, unless you like passing out and hurting your face. If you start to get dizzy, then you probably should have stopped a little earlier.