I understand that checks become "stale" after six months but that the bank can still decide to honor them. My question is if it is legal to have "void after 90 day's" on a business refund check. Our business has to send out refunds monthly, many of which are never cashed. Would a bank refuse to cash a check with a statement posted and if so, does this negatively effect our business, i.e. does it then become a "bounced" check?

At a minimum, adding such a legend to your checks should prompt payees to negotiate them sooner. It could have a chilling effect on third-party banks accepting the checks for deposit once that 90-day window is closed, since those banks would not know whether the checks would be honored when presented to the drawee bank.

Your bank is likely to take a dim view of such a legend, however. In today's high-volume check processing environment, check dates are rarely looked at, and your bank probably will not be willing to say it will bounce checks that are presented after any particular "age" is reached. I can imagine that some banks with positive pay systems (overly simplified: the bank has a list of the checks issued by its larger business accounts, and won't pay items not on the list) might be able to catch checks that are older than three (or six or twelve) months old, and bounce them. You should ask the bank about its abilities in this area.

We have some customers that use that legend, but we do not guarantee to honor their requests - if an individual brings in a check to cash and we see it, then we usually pick it up, but if a customer brings in a large stack of checks and deposits them - we won't..