Here's some recent research on masturbation frequency (and other sexual behavior) among two African groups. It's interesting because it's a reminder that masturbating with great frequency wasn't "normal" for (these) hunter-gatherers.
ABSTRACT Few systematic studies exist on the sexual behavior of hunter-gatherers and rural central Africans. This study examines the reasons for having sex, the frequency of sex (coitus) per night, sexual practices during the post-partum sex taboo, and beliefs and practices regarding homosexuality, masturbation, the use of sexual stimulants and a variety of other sexual behaviors. Thirty-five Aka and twenty-one Ngandu adults who were or had been married were interviewed. For adults 18-45 years of age, the average frequency of sex per night was about three times among the Aka and two times among the Ngandu. Age had no impact on the frequency of sex per night. Aka averaged two days and Ngandu averaged three days between days with sexual activity. Aka and Ngandu cultural models or reasons for having frequent sex emphasized their desire for children rather than pleasure. Homosexuality and masturbation were rare or nonexistent in both groups. Aka men either did not believe in the post-partum taboo or if they had this belief they did not seek out other women during this period; almost all Ngandu men said they believed in the taboo but did not adhere to it and sought out other women. Aka men had the greatest knowledge and most frequent use of plants as sexual stimulants.
African Study Monographs, 31(3): 107-125, October 2010