Answer:
In these studies, on average, 50 percent of couples conceived by the first three months; 75 percent of couples conceived by six months; and ~85-90 percent by 12 months of regular intercourse. The ~10-15 percent of couples that did not conceive within one year represents the prevalence of infertility in the population.
Infertility is usually not absolute; there are varying degrees of impairment. In those studies, 1/2 those who did not conceive in the first 12 months conceived in the following 12 months. Those that did not get pregnant within this two year interval however had a very low chance of getting pregnant in subsequent months thereafter.
If you have not gotten pregnant by 12 months of regular intercourse, then a basic infertility evaluation would certainly be recommended. You may very well get pregnant in the process of the evaluation or preparing for treatment.
The definition of infertility has recently changed in women over the age of 35 years - to six months of regular intercourse. This was changed to recognize the need to diagnose infertility and start treatments earlier due to declining age-related success rates, particularly in women approaching or older than age 40. In patients of any reproductive age, an evaluation can be performed even before attempting conception if there are obvious risk factors for impairment.