How much is my life span shortened by when I'm obese?

Too much. No one wants their lifetime shortened, so anything we can do for ourselves to help us live a long life with good quality is important. The number one action we can do is to lose the extra pounds. Being overweight, and especially obese, can increase the risks of developing heart disease, diabetes, cancer, and alzheimer's disease. All these can shorten your life and decrease quality.
Significantly. Obesity affects many bodily systems known to shorten life span. It contributes to sleep apnea, diabetes, stroke, hbp, cancer, depression, social stigma, poorer life satisfaction and increased physical and emotional stress; it affects quality of life significantly as it im. The costs of obesity are very hard to measure. Good news, losing 5% of your body wt can increase life span significantly!