Two years. I had downloaded Quicken several years ago but found it very confusing, and it did not correctly account for all my credits and debits, so I got rid of it. For two years, once a year, I downloaded my bank files in spreadsheet format, enabling me to sort and analyze them in multiple ways on Excel. I also did projections and modeling. These data enabled me to build a fairly accurate retirement budget spreadsheet which I now use to analyze my expenses on a monthly basis. I intend to continue to do the monthly expense analysis for the next few years, until I am out of the Retirement Risk period. Doing it monthly allows me to adapt my spending if I am exceeding projections. Of course spending will vary significantly from month to month with annual payments, so it's important to look at one or two complete years. As Hermit notes above, moving introduces all sorts of one time expenses. It can also affect ongoing expenses. I did not begin my analysis until things had settled after a move. I am pretty sure that once the dust settled, the move decreased my ongoing expenses (and taxes).
