Something like 75% of healthy couples with no pre-existing conditions conceive within three months of trying. This increases to around 85-90% by the end of 6 months, and 95% by the end of 12 months.
If you're under 35, most doctors won't see you until those 12 months are up, because of the high chance that you'd conceive naturally. If you're over 35, they will usually see you after 6 months of trying, because of the time pressure.
You should definitely go to the doctor earlier if you suspect that anything is wrong - e.g. multiple miscarriages, mid-cycle spotting or bleeding, or pain. You should also see the doctor earlier if your cycles are very long or very irregular, without a good reason. When I came off the BCP, my cycles were between 45 and 90 days initially, and my doctor said that they could take up to a year or so to become more regular - if they were still irregular/long after that, I should see him again for tests.