Interesting question, and the answer is...

Interesting question, and the answer is no, they don’t "expire" within a standard time period. At any time in the future following a notarial act, the validity of that notarization can be challenged. 

One scenario is when a person or party objects to the effects of a document. If the document signer’s signature required notarization, the objecting party or parties might challenge the notarization in hopes that if it is invalidated, the document’s effects will be invalidated as well.

Even for conscientious Notaries Public, a detailed examination of a past notarial act can and does expose errors, mistakes and oversights. The Notary may be sanctioned by his or her/their state commissioning official. If the Notary’s errors and oversights caused financial harm, the Notary can also be liable for the financial damage caused.

To protect themselves against such liability, a Notary may obtain an Errors & Omissions insurance policy. Not to be confused with a Notary bond (which protects the public), a Notary Errors & Omissions policy protects the Notary in the event of unintentional errors, mistakes and oversights. Notary Errors & Omissions coverage is available in every state, for a very modest premium compared to the policy’s limits of coverage. (Literally, the cost is pennies a day… view your Notary E&O policy options. Covers unintentional errors, mistakes and omissions made on notarizations performed during the policy term.  No underwriting required.

To recap, your notarial acts never "expire." They live on indefinitely as evidence; can be questioned at any time in the future; and can be marred by unintended errors, mistakes and omissions that bring sanctions against your commission and financial liability. Notary Errors & Omissions is available to help you weather the storm if you are found to have notarized improperly.