If I am a size 40F and I get a breast reduction for health reasons . How much would they take out?

Thank you for the question. Based on your description, you may be an excellent candidate for breast reduction surgery. This operation tends to be one of the most patient pleasing operations to perform. 
“Typical” patients who present for breast reduction surgery are women who have disproportionately large breasts, causing problems such as neck/back/shoulder discomfort, postural changes, bra strap grooving, skin irritation/rashes under the breasts, and/or difficulty with activities of daily living and/or exercise etc. There may be both physical as well as psychosocial “stress” caused by the disproportionately large breasts.
Reducing breast tissue mass and elevating the breasts on the chest wall tend to improve or alleviate many of the symptoms associated with the disproportionately large breasts.
Patients considering breast reduction surgery should also consider the potential downsides (risks/complications) associated with the procedure as well.. Poor scarring, for example may be associated with the procedure. Additional surgery may be necessary in the short or longer term for a multitude of reasons.
When the time is right, I suggest that you seek consultation with well experienced plastic surgeons who can demonstrate significant experience achieving the types of outcomes you would be pleased with.
Then, I would suggest you visit a few surgeons whose practices concentrate on aesthetic surgery. Ask to see lots of examples of their work and preferably speak/see patients who have had similar procedures done. Once you have chosen your plastic surgeon carefully, it will be important for you to communicate her goals carefully as well. In my practice, I asked patients to use as many “visual aids”, such as goal photographs, during the communication process. Be careful about utilizing terms such as “ideal” ( there is no such thing), “B or C cup” and/or “proportionate"… these terms can be confusing, since they may mean different things to different people. Once you have communicated your goals, your chosen plastic surgeon will be able to give you an estimate of how much breast tissue will likely need to be removed to achieve your goals.
I hope this, and the attached link (dedicated to breast reduction surgery concerns), helps. Best wishes.

Bra sizes are not standardized amongst manufacturers so bra size does not equal breast size.  That being said, if you are a 40F it is very likely you are large enough to meet most insurance company's requirements to have your breast reduction covered and paid for.   However, a physical exam would be required to estimate how much breast tissue, which we typically measure in grams, would be removed.

If you are a size 40 F and in good health, I believe you probably would be a good candidate for a breast reduction.  Although, the amount of breast tissue that is removed in the procedure varies with each patient, if you are an F cup it would not be unusual for the reduction to weight between 600 to 1200 grams (1.3 to 2.6 lbs.) per breast.  If you have health insurance, your medical plan may consider the reduction medically necessary and may help with the cost of the procedure.  Seek out a board certified plastic surgeon with extensive breast reduction experience.  Best wishes, Dr. Lepore.