Hi Will,
I believe it builds up in the system over a fairly short period of time - maybe a week? My migraines start to reappear about 4-5 days if I stop taking it, but I bridged over from a pharmaceutical when starting it, so I am not sure how long it took to build up in my system. Sometimes 50 mg doesn't seem to be enough - I get breakthrogh on my migraine symptoms. When that happens, I take 100 mg for a few days and it seems to clear up.
Hope that helps.
Melissa