Well lets take it from SOC:
Rationale for a preoperative, 12-month
experience of living in an identity-congruent
gender role. The criterion noted above for some
types of genital surgeries—i.e., that patients
engage in 12 continuous months of living in a
gender role that is congruent with their gender
identity—is based on expert clinical consensus
that this experience provides ample opportunity
for patients to experience and socially adjust
in their desired gender role, before undergoing
irreversible surgery. As noted in section VII,
the social aspects of changing one's gender role
are usually challenging—often more so than the
physical aspects. Changing gender role can have
profound personal and social consequences, and
the decision to do so should include an awareness
of what the familial, interpersonal, educational,
vocational, economic, and legal challenges
are likely to be, so that people can function
successfully in their gender role.
Taken from
http://www.wpath.org/uploaded_files/140/files/IJT%20SOC,%20V7.pdf
