I think I just found an article to answer my question!
This is taken from this article: https://www.fertilityplus.com/faq/cm.html
It's a good read on this topic!
What cycle days should one take guaifenesin?
Just as the dose can vary a bit, so can the days. The article linked above had patients starting the guaifenesin on cycle day 5 - five days after the start of menses. This seems rather early as mucus is not an issue until about five days before ovulation - based on the fact that sperm can only live five days at best, and most pregnancies are the result of intercourse in the five days leading up to ovulation. Because of this information, it is now more common for doctors to suggest starting guaifenesin about five days before expected ovulation and continuing through ovulation day (six days total, for those with regular cycles). For those with irregular cycles, one should start taking guaifenesin about five days before the earliest day ovulation might be expected. For those taking Clomid (clomiphene citrate, Serophene) in a 5-day protocol, one can probably wait until the day after the last Clomid pill before starting the expectorant.