According to a 2013 Australian study, the most attractive beard length is "heavy stubble," which comes after about 10 days of growth. 

Fascinatingly, heavy beards, light stubble, and clean shaven were all equally less attractive than heavy stubble.

The science says that each length of beard sends a different signal. 

"Facial hair correlates not only with maturity and masculinity, but also with dominance and aggression," write authors Barnaby J. Dixson and Robert C. Brooks. 

"An intermediate level of beardedness is most attractive, while full-bearded men may be perceived as better fathers who could protect and invest in offspring," they write.

Beyond attractiveness, researchers also found that facial hair affects perceived fertility as well. The more facial hair a man had, the more masculine a woman perceived him to be — especially when she was in the fertile phase of her menstrual cycle.