When must a couple seek fertility treatment?

Depends. A couple with no risk factors or medical problems should try for about 6 to 12 months before getting evaluated and considering treatment - 6 months if the woman is 35 or older and 12 months if she is younger. However, if you have medical or gynecologic problems like endometriosis, irregular periods, previous surgery, etc, you should see someone now.
Any time. If you have a known problem, then you may go at any time. But if you are stressing out about it or have questions or just want to make sure that what you are doing is correct, you may also consult a t any time. Otherwise, we counsel patients to go through testing if <35 and trying for 12 months or if >34 and trying for 6 months.