Long Answer: It depends on where you are at in training, and also on the requirements of your program and your site(s). At my graduate program we had pre-practicum, where we did 6 months of shadowing and didactics through the clinic. You'd observe intakes, therapy, assessments, assessment feedback, etc. They have since changed the setup to be more universal (I had a great set of supervisors, though others in my cohort got shafted), so who knows what they do now.
In the second year we started our practicum placements, though the experience/rotations varied by site. For instance, I work at a school for 9 months doing therapy/assessment/psychoeducation, and then for the 3 summer months I did assessment/treatment planning/IEPs. A practicum site at a hospital may have students rotate through 2-3 different departments, it all depends. A forensic site may have 6 months of assessment, and 6 months of individual/group therapy.
Some programs may offer an assessment rotation for students who want to gain more assessment experience, while other programs may offer psychdynamic/analytic training where you carry cases for multiple years. Typically graduate programs require a minimum number of face to face contact hours, supervision hours, and support hours. We were required to present and defend a therapy case before being elligible to apply for internship, so students had to complete at least 1 practicum where they did therapy. The hardcore neuro people weren't happy with this, but I understand why they did it.
As for internship and post-doc/fellowship, it varies even more. I have friends that have the same schedule for the year, while others switched every 3 months. Post-doc/Fellowship will vary by speciality. I'll probably do 9mon, 9mon, 6mon on the clinical side, and 12mon/12mon or 6mon/18mon on the research side. I looked at other fellowships that did 4 6-month major rotations, and then they offered some minor rotation options and/or protected research time. The one year fellowships may just do 1 rotation, or they may do multiple rotations. I'm less familiar with these, as I only looked at 2 year neuro / rehab fellowships...yah extended cheap labor!
Short Answer: It varies.