When will finasteride stop hair loss?

Finasteride does not stop hair loss. It is a drug that is used for treating androgenic alopecia in men. There is no cure for this.  Finasteride can sometimes stop hair loss but it is not a permanent solution.  You need to use the medication for at least 6 months to see some benefit.

The answer to this question will depend on how effective your treatment is.  Finasteride blocks only about 60% of DHT and Minoxidil is only a slight hair growth agonist that works for around 20% of users (plus has side effects).  For more effective treatments I would recommend seeing a hair regrowth specialist who does not also do surgeries and can offer you a money back guarantee on results.  That way you know that they are incentivised only to regrow your hair.

Finasteride and minoxidil are the only FDA approved treatments for male balding in men. You are on the correct treatments. It takes 6-9 months to see real effects from these medications. The first few months are sometimes associated with shedding so hair loss can get worse before it gets better. 90 % of men achieve some degree of stabilization from use of finasteride. 40 % achieve not only stabilize but actually improve their density. Please wait at least 9 months before assessing response and deciding on any next steps. 

Finasteride takes time to see the impact. In someone 18 years old, I would expect to see results (highly likely) in 1 year so be patient.