The long term safety of Prevagen isn't known, but there is no indication that Prevagen cannot be taken for more than 90 days without an increased risk of side effects.
Per the Prevagen product information, clinical trials for the supplement showed certain aspects of cognition improved over a 90 day period. From Prevagen:
The product information says nothing about safety regarding use longer than 90 days. However, it is important to know that in 2012, the FDA sent a warning letter to Quincy Biosciences, accusing them of false advertising and failing to report over 1,000 adverse event cases.
"Prevagen contains apoaequorin, which uniquely supports brain function.* In a computer assessed, double blinded, placebo controlled clinical study, Prevagen improved certain aspects of cognitive function over a 90 day period."
We wrote an extensive article regarding the Prevagen use which can be found here: Does Prevagen Work?
We are certainly glad to hear your wife feels her memory is better. Like all over the counter medications and supplements, it is important to let her doctor know she is taking it so she can be monitored for improvement and potential side effects.