I have been taking 50mgs 2x/day (so 100mgs total) for about three years now and although I never noticed a metallic taste, I did notice that almost immediately my taste changed - somewhat drastically. With some foods - I did have a metallic aftertaste which was new and/or undecernable by my friends and family who are not taking
Topamax. Also, foods I had really liked (alfredo sauce is one that comes to mind) became truly disgusting, and others I hadn't particularly liked (tomatoes), I started to really like.
I haven't noticed the metallic aftertaste in a long time - I hadn't thought about it until I read your post honestly. I don't believe it lasted for more than a few months. All of the side effects I experienced were virtually gone by 1 year (although randomly my feet and hands will still tingle).