When is Enough Enough? -Chasing

After playing many recent Heads-Up and Sng matches. I got caught with the problem of calling, or not calling flushes and straights.

My question in need of a answer is how much should you call on the flop and or turn to chase a flush or a straight? In this case you have an open ended straight draw or a four card flush on the flop. say you have 1,500 chips how much should you call to chase for each situation?

I lost many games by calling too much and lost many hands by folding to a little bet. Also, if you have a flush or straight draw and someone min bets do you raise for value assuming you will hit? ( if your not semi-bluffing)

All comments would be apprieciated, Thanks .