A part-time Masters will generally take twice as long as a full-time Masters. A full-time Masters will usually take one year, so studying part-time will spread study over two years. Part-time students follow the same modules as full-time students but balance the modules across two years of study.
Generally credits are split fairly equally across each year. The dissertation is usually taken in the second year which often means that more taught modules (which include lectures and seminars) are taken in the first year. However, this does depend on the individual course.
Details on course length can be found on individual course pages.