How and when DHT is produced which cause hair loss?

DHT is produced by your body everyday. It is a hormone that everyone has (both men and women). DHT does not cause hair loss. Genes that you inherited from your mother or father cause hair loss.

DHT is produced in our bodies by both men and women.  The Total Testosterone in our bodies goes through a conversion process to create both Free Testosterone and to DHT.  Men produce both of these hormones at higher rates than women do, but we all have them both.
Both WebMD and The American Hair Loss Association clearly state the relationship between DHT and hereditary hair loss.  You can do a simple search for "how does DHT cause hair loss?" and see or the reputable references for yourself.
To answer your question, DHT is produced by your body everyday.  It is produced in many places including within the prostate and seminal vesicles but also in your skin and scalp.  Those suffering from hereditary hair loss inherit both hair follicles that are more sensitive to DHT than others, plus higher than normal levels of DHT within their bodies.

DHT (short for dihydrotestosterone) is made from testosterone thanks to the hard work of the enzyme 5 alpha reductase.Not everyone's hair follicles are sensitive to DHT. But in those individuals with genetic hair loss, there is good evidence to support the notion that DHT causes hair follicles to get skinnier or miniaturize. Drugs like finasteride and dutasteride block 5 alpha reductase and prevent alot of the testosterone from being converted to DHT. This in turn blocks miniaturization and in some men the speed of genetic hair loss progression.