Depends on age. A couple should be evaluated after 1 year of infertility. If the female partner is over 35 years old, the evaluation should be performed sooner at 6 months.
Anytime vs. 1 year. I believe whenever they are ready. On the other hand, physicians usually don't push evaluation until after 1 year of unsuccesful trying. When the woman is over 36 years of age, it is nowadays appropriate to start evaluation after 6 months. I'm glad you asked when a 'couple' can start, both partners should be involved from the very beginning.