Is there a regulation on this notation or is this just a notation put on the check as requested by customers. In other words, if we refuse to cash a check that is over 90 days, are we in compliance?

Unless you have a contract with your depositor that calls for you not to pay checks over 90 days old, refusing to pay a check drawn by your depositor on its account with you simply because the check is over 90 days old would open the bank to a suit from your depositor for wrongful dishonor. That is not a suit your bank should want to defend, because the damages can pile up quickly.

On the other hand, if the check is over 6 months old, the law (the Uniform Commercial Code) will allow your bank to refuse payment with impunity (or, if it does so in good faith, to pay the check).

...and if the legend is on a check that is not drawn on your bank you have no obligation to cash it anyway. Nevertheless, it would be particularly wise not to cash it if it is beyond its stated expiration date.

The customer and the drawee bank may have made the legend part of their contract. If so, the drawee bank would have every right to return it.

Have it sent for collection & have the customer pay for the fee. That gets you off the hook.

Researching an old thread here....what if we paid the check after the "void after" period and the customer wants it returned? Our account agreement says we are not required to honor restrictive legends, but does the customer whose account it is drawn on have the right to return it or would we be subject to wrongful dishonor by the payee? There was no agreement between the bank and the customer to honor the restrictive legend.

You should consult with bank counsel.

Review your depositor's agreement. Most indicate the bank may disregard any such restrictions. If the customer wants to do something like this, then they should subscribe to a positive pay system where they then can void their own authorization after the 90 day period.

The only party that can bring a suit for wrongful dishonor is the issuer of the check, and that's not going to happen. Once it's paid, and the midnight deadline has passed, it cannot legitimately be returned.

Just as you said...