90% of couples will conceive within 12 to 18 months of trying.

Infertility is defined as the inability to get pregnant (conceive) after 12 months of frequent, unprotected sex (intercourse), if you are less than 35 years old.
If you are 35 years of age or older, doctors will start evaluating your fertility after six months of unsuccessful attempts at pregnancy. If you are having regular menstrual periods, you are probably ovulating regularly. You need to know that you are the most fertile in the middle of your cycle, between periods. That is when you release an egg. You and your partner should have frequent sex on a number of days in the middle of your cycle. You can use an over-the-counter fertility kit to find out when you are ovulating. You should not use any lubricant, and the standard wisdom is that you should not get up immediately after having sex.

Somewhere around 25% of couples will be pregnant at the end of the first month of trying. About 50% will have conceived in 6 months. Between 85 and 90% of couples will have conceived at the end of a year. Of those that have not conceived, some still will, without any specific help. Many of them will not.