How long does an ms relapse usually last?

Multiple sclerosis : A relapse, also known as attack, flare or exacerbation, has to last at least 24 hours to be considered a real relapse. True relapses usually last several weeks, although they can be as short as a couple of days or as long as several months.

Depends on treatment: Use of steroids should stop progression and reverse over a period of 3 or 4 weeks at most. Untreated, about 50% of relapses will spontaneously improve, but 50% will be permanent.