How long will it take for my receding hairline to grow back if I apply castor oil to the receding area every night?

Castor Oil has not been proven to regrow hair. Start doing your research on a hair loss prevention regimen.It is important to get on a preventative regimen. You should switch to an organic shampoo. You can also consider a natural DHT blocker like Saw Palmetto. There are also medications like Finasteride (a DHT Blocker) and Minoxidil (Rogaine) that can be very helpful in maintaining you existing hair, helping to prevent future loss and even regrow some hair. Before taking any medications you should discuss this with your general physician.

Castor oil is not going to help a receding hairline. You will want to see a physician who specializes in hair loss to determine if your hairline changes are from hairline maturation or androgenetic alopecia (or something else). 

I don't believe that you will see any impact of Castor oil on your hairline recession. See a doctor and do it right