It varies enormously. The pass rate on the first try for the hardest of the four exams is extremely low. The process is usually done in conjunction with acting as a professional. You can't act before the office, but the archetypal journey to becoming a patent trademark agent is that you get picked up by a firm that pursues patent and trademark work. You essentially stage—you work and you do your exams—and then eventually you get your licence. It depends on how much work you're doing and how much time you have to dedicate to the exam process, but it's usually a couple of years to be able to become an agent.
On November 5th, 2018. See this statement in context.