Definition of infertility

Infertility is 12 months or more of unprotected intercourse without pregnancy.
Primary infertility: Infertility without any previous pregnancy.
Secondary infertility: Fertility problems occurring in a couple that has conceived on their own and had a child in the past.
Sterility: When there is no chance for a pregnancy. This is different from infertility which represents a reduced potential for pregnancy.

Most childless couples with a female age under about 43-44 that are having problems getting pregnant are considered to be infertile but not sterile.
When to see a fertility specialist?

In general, it is appropriate to see a doctor for medical assistance for infertility after 12 months of trying to get pregnant on your own. Many couples will start the infertility workup process with their general gynecologist, while others prefer to go straight to a fertility specialist. Either place to start is appropriate, with the general advice being to get to a fertility specialist relatively sooner if the female partner is 35 years old or over.
Learn more about female aging and infertility issues

The appropriate amount of time to try on your own can be longer, or shorter than one year. For example, if you are only 25 years old and feel that you want to give it more time to occur naturally you might try for another 6 to 12 months. You also might want to seek medical help for fertility issues without waiting for a year of trying on your own if you have a condition that is a known risk factor for infertility, such as irregular menstrual cycles, endometriosis, previous tubal pregnancy, PCOS - polycystic ovary syndrome, etc.

It may also be prudent to seek medical help for infertility problems before 12 months of trying if the female age is 37 or older. For couples with a female partner age 40 or older, it is appropriate to see a fertility specialist if not pregnant after 3-6 months of trying to conceive. A very high percentage of women over 40 will have fertility issues related to age and egg quality and will need medical help to get pregnant. If the female partner is 42 or older, it is recommended to see a fertility specialist right away when wanting to get pregnant.
Fertility advice

You do not need to have sex all the time in order to get pregnant. If you like it that way, then no problem - it doesn't reduce a couple's fertility potential either. The best fertility advice in terms of frequency of intercourse is - every day or every other day "around ovulation". Ovulation is usually day 14 if the menstrual cycle length (day 1 to day 1) is 28 days, or day 16 if the cycles are 30 days apart. In other words, ovulation usually occurs 14 days before the next period is coming.

The egg only lives about 12-24 hours, while the sperm (if normal) will live in the female's reproductive tract for up to 2-5 days - while maintaining the ability to fertilize the egg. Therefore, the best fertility advice would be to have sex on the day of ovulation, or the day before ovulation (or both). Having sex more than about 24 hours after ovulation should be fun and all that, but it isn't likely to increase the planetary population.