How long does it take for a birth parent to find their child?

How many years do most people search for their child? I have been searching for my son who was born on September 8, 1985 now for 4 years. I just started searching for my son born on May 26, 1989. I have waited until they turned 18 yrs. before I started searching. I see how many mothers are out there searching. I have also noticed how many girls are searching. What do you do if your child isn’t searching? My brother has never searched for his birth parents and he is 42. Is the desire for girls more then boys? I’m not really sure of any of these answers, but would like to know. I would like to know that they are ok and happy. How do you find a child who is not searching?