How long after I stop taking a med that caused significant hair loss (topomax) will the hair stop falling out?

I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

There are dozens of medications that are either associated with hair loss when you start, or when you stop. There is no absolute pattern. In many cases the loss starts 2-3 months after stopping but this varies considerably as well as how long the loss will continue. Propecia (Finasteride) is not usually prescribed for women.   

It is unusual to lose half your hair from a medication other than a chemotherapy. You should definitely seek further evaluation by a dermatologist to look for multiple causes of hair loss at your age. Oftentimes female pattern hairloss is unmasked (made more obvious) by compounding factors like medications, thyroid disease, anemia, or hormonal changes. It could be there are more things going to blame for your hairloss.

I am sorry to hear about your significant hair loss. I have not seen significant hair loss related to taking topiramate(Topamax) so I would be sure to see a physician board certified facial plastic surgeon or dermatologist experienced in hair loss issues in women to make sure there are no other underlying conditions causing your hair loss such as a low thyroid level causing hypothyroidism. 

If an individual does have drug induced hair shedding (telogen effluvium), the shedding starts 2-10 weeks after starting the drug and can last 9-12 months. For a small proportion, shedding can persist even when the drug is stopped.Please see a dermatologist for guidance. If androgenetic alopecia is present or a telogen effluvium has unmasked androgenetic alopecia, finasteride is appropriate. Otherwise it is not an appropriate treatment for a drug induced telogen effluvium.

The time it takes for the shedding to go away varies by medications, your genetics and other health factors. It could take a year and you might find that it will not reverse.