1. As you want.
2. As you want
:-)
In fact, i think it is not really matters. If you have constantly sweeping radar on some area, measuret in thousands of miles, you can spend another hour or day to discover base.
So, lets say 15-30% chances per day is normal. I think.
But maybe other modders will want to choose chances by ruleset.
By the way, how it works with crafts? 100% once in a hour?