OOH... I don't know that there is such a widely applicable "rule of thumb" for meads. In general, good "wine strength" (i.e. 10-14% ABV) meads will age similarly to good white wines, peaking at about the same time, but in my experience they both hold their peaks and maintain their drinkability, for longer intervals.
In my experience, traditionals will keep for many years, and they will get generally "better" over the course of the first 1-5 years. They hold for an indeterminate while after that.
Melomels are all over the map. Depending on the antioxidant properties of the particular fruit used (as well as on the handling and bottling technique employed by the meadmaker), they can do anything from peak in 3-6 months, to 3-6 years.
Metheglyns are even harder to forecast, since some herbs and spices start to lose presence in a matter of months, while others can get apparently stronger with age.