That's ridiculous. All drugs are approved based on limited duration trials and virtually all non-oncology drugs are approved for indefinite use.
Very occasionally there will be time limitations in the label - in fact one approved migraine prophylaxis drug (methysergide) has a six-month limit in the label (can take it again after a drug holiday). But nowhere in the Topamax label is there any mention of a time limit. In fact people with epilepsy might take it their entire lives.
Bottom line, Topamax is indeed approved for indefinite use for migraine (and of course for epilepsy).