That depends on numerous factors both male and female.
As a woman ages, she has less chromasomally normal ova(eggs) and therefore,as she ages, it becomes more and more difficult, possibly taking years to conceive.
% of chromosomally normal ova:80%
This is because of the effects of gamma radiation causing chromosomal damage to the ova(eggs)
Miscarriage is a normal process to assure that over 98% of all babies are born perfectly normal. A miscarriage happens because of chromosomal abnormalities.
An ultrasound at 10 weeks after conception with a fetal heartbeat reassures that there is a 95% chance of the delivery of a perfectly normal newborn.
Infertility is defined as...
Most infertility is a combination of suboptimal fertility factors in the man and suboptimal fertility factors in the woman.
A decreased sperm count in the man, or a prostate infection, combined with inadequate conception lubrication in the woman can create long standing infertility.
Optimizing all fertility factors will provide maximum chances to conceive.
Even women who have failed multiple IVF procedures have a 17% chance of spontaneous conception over four years.