There's no wrong or right amount of time to wait before going to see your GP, but a lot will depend on your age and personal circumstances. If you're a young couple under 35 and have no reason to suspect problems, (for example, previous surgery or irregular periods), However, women over 35 would be well advised to seek help earlier, as the chances of conceiving drop rapidly after 35.
The usual advice is to seek help after about a year of trying to conceive.
Trying to conceive can be very stressful and can lead on to feelings of anxiety and depression, as the months pass by without a positive pregnancy test. Your GP can carry out a few basic tests on the NHS to rule out obvious fertility problems. Progesterone monitoring, rubella assessment, Chlamydia screening and semen analysis should be performed in line with current guidelines before referral to fertility clinic, preferably in a primary care setting. Your GP can organise these for you and there's no set time to wait before you'll be allowed these basic tests. It may be necessary to refer you a fertility specialist.1
1National Collaborating Centre for Women's and Children's Health. 2004. Fertility: assessment and treatment for people with fertility problems. London: RCOG Press