I originally had the schedule for my macro set to wait 10 min, run every 15 min, but I was having the same problem as terrypin that the macro never ran, so I shortened the time to 2 min / 1 min for testing. As I understand from the explanatory material, "After the computer has been idle for m minutes, run every n minutes" means that in the absence of keyboard/mouse activity the macro will run at (m+n) minutes, (m+2n) minutes, (m+3n) minutes, etc. Thus I did mean 3 minutes in my closing question. (Actually, it would seem to make more sense to have the scheduler run the macro at m, m+n, m+2n... minutes, but I suppose that's another issue.)
My computer was not absolutely idle during the wait period in the sense that there were various background programs running, but nothing, as far as I know, that simulated keystrokes or mouse actions. Since terrypin's 2nd macro does simulate keystrokes I would expect a 3-min repeat time for it, based on a 2 min / 1 min schedule. My macro calls a Visual Basic executable which writes to a file, but I didn't think writing to a file constituted keyboard activity.