How much does a womans fertility decrease when having endometriosis?

A lot. Endometriosis is present in about 3% of women, but in fertility clinics about 60% patients may have it. Therefore endometriosis can definitely affect fertility. Endometriosis can cause adhesions, tubal blockage, defects with ovulation, increased inflammation, decreased egg quality, etc... Surgery for endometriosis, more specifically endometriomas, can also affect your ovarian reserve.
Depends on age. Up to age 26, not much. Beyond that pr's decrease steadily but respond well to treatment up to 33-35yrs. Beyond this age the rates dramatically decrease to nearly zero at age 40. We believe the cause is premature aging of the ovaries with loss of eggs. Best treatment is laparoscopic excision at a young age, preferably under 25 years. No long term data yet.
Depends. Endometriosis can have a significant impact of fertility. It can affect the fallopian tubes, create scar tissue and affect your eggs as well. So it is very important to speak with a fertility specialist about your endometriosis, sometimes even before you are thinking about becoming pregnant to plan ahead.