What must I do to reach certainty about a course of action?

Suppose that I'm being careful in my thinking about a practical matter -- perhaps about how to solve a problem at work, whether to move to a new city, whether to marry my girlfriend, whether to cut contact with a problem friend. When can I say that I'm certain -- or at least justified in acting on my conclusions? Given my personality type (INTP), I tend to leave questions open for far too long, when really, at some point, I need to close them. Are there any general guidelines or principles around figuring out what that point of closure should be? Even then, when should I revisit my conclusions, if ever?