It's hard to say in my case, honestly. I began having wicked 'headaches' or 'migraines' in 2000/2001. Chalked that up to the building I had been living in at the time and something in the building materials. I felt fine after I moved out.
Jump ahead to early 2004, I had a laposcopy, found severe endrometriosis, surgery... then I was hit out of the blue with what I now know is actually Trigeminal Neuralgia. <~ this was probably the point when I first thought to myself "Okay something's not right with this..." I felt like I was going to have a stroke, if not, had one already.
Moved, TN attacks subsided, guessed building materials again. Moved into house in 2005, spent the next couple years with symptoms increasing, namely pain/pressure around my left eye. I thought it was due to weather changes. I was in a world of pain, missing a lot of time at work, etc.
Then I woke up in January 2007 freezing cold, and shaking (maybe spasms the way it presented). I couldn't control my body to stop it, so I got into warm water (which actually helps me, unlike many with MS), got warm, went back to sleep. Woke a few hours later drenched in sweat. This was the point where I finally said "Enough is enough" and I went off work, on short and then long term disability, to an eventual MS diagnosis (it wasn't as easy as I remember it now, the first neuro I saw fluffed me off completely, I went through a dental pain specialist and several doctors before an ENT paid attention to what was going on and got me the right doctors to help me).
So Cliff notes: I believe my onset symptom of MS were those Trigeminal Neuralgia attacks back in 2004. I was diagnosed in March 2008, my neuro suspected I'd been living with MS for at least 5 years prior to the diagnosis.
2004 to present - Trigeminal Neuralgia
2007 to present - Burning Mouth Syndrome
March 2008 - Multiple Sclerosis DX (Treated for 'relapse' at that time)
05/2008 - Relapse
05/2008 to 02/2009 - Copaxone
10/2011 - Relapse - Optic Neuritis developed
9/2012 - Relapse - Balance issues 1 sided
8/2012 - Erythema Nodosum, diagnosed in 10/2012