I've been taking exactly the same dose as you for about five years now. The effect was very gradual over about five or six months, but comparing before and after the difference has been profound – before, a 20 minute conversation would leave me exhausted for days, now, I talk all the time to carers, friends, and my computer! – although I do remember being told at the time that it only benefits about 30% of MS patients.
It's also worth remembering that it's a useful antiviral. In fact I think its benefits to MS were discovered as a by product of its use as a flu preventative, when MS patients began to report an increase in energy levels.
If I were you, I'd stick with it a bit longer!