How long will a migraine and other side effects last after stopping mirtazapine 15mg?

Depends : Recommend slow reductions over 1-2 weeks if been on < 2 months and perhaps even slower withdrawal over 6-8 weeks if you've been on it longer. You may wish to have further discussion with your gp in consultation with mental health re best way to switch

1-2 weeks: People often develop symptoms when they suddenly stop taking medications like mirtazapine. In the same way that it takes a while to be clinically effective, it also takes a little while to leave your system.

Mirtazapine: Usually most side effects other than headache and nausea will abate in less than 24 hours. Most side effects that take many days to weeks before they shot up take longer to resolve as compared to those like sleepiness that occur after a single dose typically and should resolve in < 24 hours.