i'm still working on recovery from my first attack in january. stupid hands won't get all the way better. so i know nothing about average times between relapses.
significant percentages of people with ms have been found to have low uric acid, low b12, low D... low uric acid is particularly relevant to the relapse condition... i don't know what ideal uric acid levels are, but for b12 >= 500 is the bottom end of the normal range in japan, and my doctors have me aiming for 125-150 nmol/l of serum D (at last count i was up to 72!!??!!)
so, maybe a baseline test for all three and annual checks thereafter, more often as needed if correction is required?