fertile for how long..

[rosewein.livejournal.com]

So I'm not trying to get pregnant, or even trying to prevent pregnancy i just tend to want to know things. I'm curious how long should I be producing fertile mucus? is there an average?

Also while trying to Google this answer i discovered a page promoting Robitussin as a product to aid in increasing fertility this seems odd to me. anyone understand why something designed to remove mucus would help?
http://fertilityplus.com/faq/cm.html