For those seeking a definitive, clinical assessment of what a relapse is, i would draw your attention to the very nature of MS, and the extraordinary variability that permeates every single aspect of the condition;
the symptoms; the severities; the age of onset; the prognosis; the efficacy of this, that or any other medication; the effects of behaviour / environment / genetic inheritance; basically anything and everything (aside from the fact that the illness sucks big old bawls) is so varied, it can be said to be tailored to the individual.
thus, the annoying non-answer to questions such as, “how long does a relapse last?” is the ever dependable, “how long is a piece of string?”
it’s the not knowing which proves to be the greatest ‘unraveller’ i think.
in order to try to derive some sense of what might be going on within me, i usually take the professionally sourced expectations with a pinch of salt, and indulge my ‘inner vibe’ instead.
when ‘things’ (whatever they may be) start to feel ‘different’ to normal (whatever that might be), i take note, usually in a journal. New symptoms are listed and i make sure to note the ‘things’ that resolve / improve just as eagerly as those which do not.
it becomes easy to see, whether ‘things’ are accumulating or falling by the wayside. and of course the passage of time is dutifully recorded also. a fuller picture can be derived from recording any activities performed or meals consumed.
i understand that a relapse needs to be something ‘significant’ (whatever that might be), which lasts for more than one day. distinct relapses can be counted if such significant events occur more than 30 days apart.
for what it is worth (from my own experience and nothing more), my last relapse lasted six weeks; that is that new symptoms introduced themselves almost daily for about four weeks; for the final two weeks, severities varied and some body parts recovered. basically, ‘my inner vibe’ at the six week stage told me that nothing new or nastier was happening. and so it was. that relapse was during the summer of 2014 and i still have remnants of some of the symptoms which first struck me. i doubt they will ever go away.