Additionally, is it true that, as Wikipedia claims, "As late as the seventeenth century in Europe the practice was commonly employed by nannies to put their young male charges to sleep?" (From the article on masturbation)
The masturbation as sleep-aid tidbit is true. We even have a woodcut of the Holy Family with Anne masturbating baby Jesus. Sergio Bertelli talks about this is his chapters on royal genitalia in The King's Body.
In terms of "medieval serfs" having time to masturbate...when do you have time to masturbate? They would have squeezed it in (out?) when they wanted to and could. And yes, medieval people did masturbate. Generally, outside of the Church, medieval people were far less squeamish about sex than anglophone people now are.
In Pillars of the Earth, Ken Follett portrays sex as a very public thing; the mother of Jack obviously masturbates while sleeping in the same area as her son, and in the cheap inns, he describes multiple couples getting it on while children peed on the floor. How accurate is his portrayal of medieval norms?