What percent of men and women masturbate? and at what frequency?

According to Masters and Johnson in their book 'On Sex and Human Loving'
third printing, page 295:

Data about masturbation are a bit tricky to interpret. You may
recall that Kinsey and his colleagues found a wide discrepancy in
the incidence of masturbation between male and female adolescents,
but some recent studies suggest that this difference may be
narrowing (see chapter 6). A similar trend may also be occurring
in regard to masturbatory behavior in adulthood.
The kinsey reports stated that 92 percent of the males and 62
percent of the females queried had masturbated at least once in
their lives. More recently, two separate studies came up with very
similar statistics: The _Playboy_ survey (Hunt, 1975) found that
94 percent of 982 males and 63 percent of 1,044 adult females had
masturbated, and Arafat and Cotton's study (1974) of 435 college
students found masturbatory experience in 89 percent of males and
61 percent of females.
However, Levin and Levin (1975), summarizing data from a
_Redbook_ questionnaire survey answered by 100,000 women, found
that almost three-quarters of the married women had masturbated
since marriage. Providing additional evidence that more women seem
to have tried masturbation today than in the past decades, Hite
reported that 82 percent of her sample of 3,000 women had
masturbatory experience.

For more details and speculations as to why the rise, techniques used,
frequency, etc -- buy the book.