How Long Does It Take For Betahistine To Work?

How long does it take for betahistine to work?I am 59 years old, 5'4" and 160 pounds. I suffered a migrain that made me very ill. I am one who never is sick to thier stomach and was violently sick about 8 times in a period of 6 hours. The following day I couldn't sit up , stand up or walk without nearly falling over. It seemed the worst was when ever I looked to my right. My question is: How long until the 8mg twice daily betahistine starts to work? And is this going to be an ongoing problem?