For How Long Should Valance OD 500 For Migraines And Headaches?

I'm 34 years old n suffering frm migraine frm last two years. I have been taking valance OD 500 daily at night . In between I have tried to leave the medicine but then the migraine starts again. Though the situation is under control but still in between I have minor headaches . I have got my brain MRI done. The reports were ok . For how long I will have to take medicine? Is it safe ?