How much weight will one have to lose before loose skin becomes a problem?

Depends: The laxity of the skin after weight loss depends on age speed of weight loss and you natural skin elasticity. While your skin may be looser, it is still a good idea to loose weight if you are above your ideal weight as this may help with medical issues.

Depends: On your current weight and the condition of your skin. Stretch marks may cause the skin to be loose with very little weight lose. The older you are the more likely it is to sag as well. I wouldn't worry about this it is far better for you to be at a healthy weight than to avoid losing weight to keep from having saggy skin. It will sag regardless as you age. Consult a plastic surgeon if it sags.