That varies a lot from person to person. For my daughter it was almost instantaneous because she had so many lesions and attacks in a short period of time. For others it is not so obvious. They may have symptoms, but no supporting evidence on MRI or from a lumbar puncture. Sometimes it takes a while for the diagnostic tests to finally pick something up. Unfortunately, some people go for years in Limboland with either a 'possible' or 'probable' diagnosis. Some people don't even get it that nailed down for a while.
Sorry there's not a better answer for you. I'm sure this isn't what you wanted to hear.