I have two teenagers on Nortriptylin. My son was diagnosed 4 years ago with Basilar Migraines and my daughter just last month with migraines. (We're still trying to figure everything going on with her - auto immune issues included with hers.)
I can tell you that with both of them the first the first week or two they noticed the headache pain was lower -- but still there. It took well into the first month to completely get rid of the migraine & rebounders they had been having.
Son was on for 18 months & went off for a year; however, he was going into school and couldn't afford to miss 4 days a month to a migraine so he went back on.
Hope you find it helps to kick the danged headache out!