There's not necessarily one answer for this. Since the program's reports and calculations are on a year-by-year basis, I try to update first based on year end values. In late January/February, there is typically a software update that includes the "new" tax rates, Social Security COLA, etc. information for the new year so I use this as my baseline update. During the year, you can adjust/optimize/stress test/perform "what if" analysis or add/delete other factors that are relevant to your specific situation.

Since there are always variations in returns (among other factors), you may want to add in a safety factor to have your actual spending a bit less than the "ceiling" that ESPlanner recommends, in case things don't go as planned. Also, in case of large market drops such as 2007/8, you can model these through "what if" analysis to see how big a hit your results will take under adverse scenarios.

Best,
Brian