If we treat four consecutive years as a unit (quad-year, let's call it), then one quad-year comprises 4 x 365 + 1 = 1,461 days, which is not divisible by seven.  By putting seven quad-years end-to-end, the resulting 10,227 days is divisible by seven and covers all possible calendars.  Seven consecutive quad-years will repeat themselves exactly every 28 years. 

As a finite domain, the puzzle can be solved by determinism.  Your spreadsheet is a handy tool for carrying out an otherwise tedious process, and here are the results -- 28 Magnanimous Months, alongside the number of months in between...

 

July, 2011 March, 2013 August, 2014 May, 2015 January, 2016 July, 2016 December, 2017 March, 2019 May, 2020 January, 2021 October, 2021 July, 2022 December, 2023 March, 2024 August, 2025 May, 2026 January, 2027 October, 2027 December, 2028 March, 2030 August, 2031 October, 2032 July, 2033 December, 2034 August, 2036 May, 2037 January, 2038 October, 2038

19 16 8 7 5 16 14 13 7 8 8 16 2 16 8 7 8 13 14 16 13 8 16 19 8 7 8 8

This pattern will repeat itself ad infinitum.  By inspection, then, here is the solution to the puzzle...

The times between successive Magnanimous Months range between 2 months and 19 months.

Epilog

Twelve years went by after the publication of Magnanimous Month.  Your puzzle-master received the following message:

This coming February cannot come in your life again. 
This year's February has...

4 Sundays
4 Mondays
4 Tuesdays
4 Wednesdays
4 Thursdays
4 Fridays
4 Saturdays

This happens once ever 823 years.  This is called miraclein.

The message goes on to promise miracles for immediate forwardings to others. 

Solvers recognize at once that every February has exactly the same four-of- each weekday distribution except in leap-years.  Indeed, every month of every year has at least those same quantities plus bonuses of 0, 1, 2, or 3 days-of-the-week.

By the way, the word 'miraclein' cannot be found in 55 dictionaries.