FS recommend 12 months if you are under 35, six months if you are over.
This. However, if you have a prior diagnosis of a condition that affects fertility (eg PCOS, endometriosis), you can get a referral after 6 months, even if you're under 35.

Also, if you start charting your cycles (as per Andie's suggestion) and you identify that you are not ovulating, you can go sooner. Three annovulatory cycles in a row would be sufficient justification. That's what my sister did. If you want to start charting (and I do recommend it), the charting course on Fertility Friend (website) is an excellent place to start learning what to do. The website also provides a charting function and they have a free iphone app for charting too. The basic charting functions (and the course) only require the free membership.