I know it has already been answered but..
2nd wind, can be used in the following ways
: As a Dwf, as a minor action at any point
: As anyone else, standard action at any point
: It can only be used once during an encounter
: It allows you the same benefit as Total Defense
: Certain skills and powers allow you to spend you 2nd Wind or allow another character to spend it outside of the above circumstances
There is no amount of HP you should be at but typically it would be wise advise not to use your 2nd Wind outside of being bloodied, after all it would be a waste.