You can become “proficient” with Zbrush as a tool, in anywhere from a few weeks to a few months, depending on how much effort into becoming proficient with it.
It can take years of experience to become familiar with all the associated areas of expertise that it takes to really understand and excel at digital 3d. Keep in mind many of the most impressive images you see on this site are actually produced with a combination of tools and disciplines.
It generally takes people a significant portion of their lives to learn to be good artists and sculptors, and being proficient with Zbrush alone wont make you one. It is only a tool.