How long should a couple, who's trying really hard for a pregnancy, wait before visiting the fertility specialist?

Depends on your age as Hui Qun suggested. Gynaes suggest women below 35 and above to seek help after a year trying. Above 35 after half year of trying. I do have friends married in their late 20s who thought they have time on their side, so by the time they seek help from the gynaecologist in their mid 30s to late 30s, their chances of pregnancy through assisted reproduction is much much lower than the chances of pregnancy of a 20 something year old who also goes through the same kind of assisted reproduction. I would advise those who really want children to explore their fertility and reproductions options early, don't drag. The younger you are, the higher chance you will produce a offspring through natural (if lucky) or artificial means.