You can start Part II anytime you want after you've completed part I. You will have 1 year to complete Part II (from date of assignment purchase/download). Part III should then be scheduled immediately after you have uploaded/submitted your Part II assignment. Assignment grading will not commence unless you've completed part III also. (Parts II and III will be merged for grading)
But with SCEA 5 on the horizon (and the old SCEA nearing its End of Life), I wonder how that will change the deadlines?