This system is mostly borrowed from Charles Krigbaum and Daina Volodka Staggs.

Pre-Twinkle = 30 minutes
Book 1 = 30 minutes
Book 2, add reading component = 45
Book 3 = 45
Book 4 = 60
Book 5-8 = 60
Book 9+ = 90

And, though every person is on their own path, I choose to visualize that up until Book 4 most students are in the book that their grade is. 3rd graders are in Book 3. 4th graders are in Book 4. After that, the books start accelerating.

As far as pacing, I generally feel that Pre-Twinkle phases will take a year, Book 1 will take 2 years, and then each book afterward will take no more than a year. Keep in mind that this pace holds through summer months and books with denser repertoire.