Unfortunately your asking a 'how long is a piece of string' type of question and difficult to answer without generalizations....basically each PWMS is completely unique, whilst your MS story may be similar to others its not the same as anybody elses, and nobody knows what where and when the next lesion will be so it might be easier to get your head around this news if you look at DMT's for you as doing exactly what they are designed to do, which is to slow down an unstoppable disease.
Unstoppable because currently there isn't any known cure that can stop it, and there needs to be a cure or it will continue to progress. The best you can say is 'slow down progression' and that is huge, mega, really brilliant news because theres still only something like 33% of MSers that find a DMT that actually works for them...
Hope that helps a little.......JJ