Why should I do a MAF test every month (and why not more or less frequently)?
Doing a MAF test every month lets you track whether your aerobic system is improving on a regular basis. Testing daily (by using a GPS watch every run and tracking your mile splits) is useful, but doesn’t tell you that much about your aerobic development on a day-to day. While your body’s performance readiness fluctuates several times a day, the aerobic system improves on the scale of weeks and months.
For example, if you had a more stressful commute home, your daily MAF pace might slow down. If you had a bit more greens than usual (helping along your digestion), your daily MAF pace might speed up. But this doesn’t mean that you got significantly fitter from one day to the next. It means that on the day you ate greens, your body was better positioned to make use of the aerobic machinery it already possessed.
And if you don’t do a MAF test every month, you might not realize until 3 or 4 months down the line that the training volume you’ve been doing has been too light (which is one of several reasons MAF speed doesn’t improve), or too heavy (in which case, your MAF speed will have either not improved or dropped). Testing your aerobic system on the timescale on which it develops (a.k.a. monthly) is the best way to know you’ve been improving.