How long will my hair be falling out after stopping birth control? How can I help stop it?

Many medications may cause hair loss but it is generally not the number one cause of hair loss in women.  You may want to see a doctor to find out if there are other cause for your hair loss.

Hair loss often occurs in women who start and stop birth control. This typically occurs 1-2 months after starting and stopping and can last 4-5 months. For some individuals it lasts 9-12 months. 
For the vast majority of individuals, the abnormal shedding eventually stops and returns to normal shedding patterns- even without treatment. However, some women (small minority only) develop a chronic shedding pattern for an extended period of time and some notice that density does not make it back fully on account of an acceleration of underlying androgenetic alopecia.
In summary, most women will experience additional hair shedding for a few months after starting and stopping birth control. The excessive shedding will eventually slow and return to normal for most.
Consultation with a dermatologist is advised if shedding persists after 6 months.

A complete work-up is a good idea. I have written about this for a woman who was experiencing considerable hair loss (see below)