The integrated school I went to earlier this year used to quote 12 weeks for the IR training phase. Mine took 18 weeks. I think integrated schools at the moment will be nowhere near their advertised timeframe. 20 months seems to be the minimum at the moment from zero to fATPL. Far too many students being squeezed through the door, and nowhere near enough instructors to deal with them.
When I first started the IR phase, my instructor had three students. When I finished the IR phase, he had nine students. This was caused by a combination of instructors leaving, the inability to recruit, and the increasing number of students.
Instructors used to do two flights per day, or four sims. So that worked out at 3-4 days between training sessions if he had nine students. Especially when the students closer to their IRT used to get extra attention. The maths seems legit, and I imagine one session per week is correct.