I follow these three golden rules:
1. When asked.
2. In a life-threatening situation.
3. When I feel that the person is unable to take a right decision (And I care).
Are these rules correct? I don't know! There is no right answer for this. Most of the people I know offer unsolicited, unwanted, and unwarranted advice, which often is detrimental and is also not appreciated most of the times.
On the other hand, following these rules makes people feel that I am aloof and disinterested, which is not true.
To each, his own!!!
EDIT: Realized it yesterday, that most of the people do follow similar set of rules. The only difference is that, others assume that people are unable to take a right decision till proved right, and I assume that people are able to take a right decision, until proved wrong.