Due Diligence is the act of sending out letters to your lost owners or publishing their names in newspapers. Almost every jurisdiction has laws that require companies to perform due diligence. The most common requirement is to send letters 120 days to 60 days before your filing is due. You'll definitely want to meet each state's requirements but I believe you should do more.
Many of your lost owners are likely customers whose business you wish to keep. I encourage you to send a round of customer service letters as soon as you can. The earlier you reach out to the customer, the better your chances of making contact and renewing your business relationship.
You'll still be required to perform due diligence within the states' required timeframes, which will give you a second chance to reconnect with the remaining lost owners.
Whatever you do, do not ignore the requirement to perform due diligence. Many states now have a sworn affidavit on their coversheets stating that due diligence has been performed. You definitely don't want an officer of your company signing off on something you did not do.
How do the states know if you have performed due diligence? Many times, it is quite obvious. Most states also have a requirement to send letters to owners and/or publish their names in newspapers. If they find a large percentage of the owners you could not find, your due diligence will come into question.
So please do the right thing for your business, for your customers, and for the states. Everyone wins when due diligence practices are followed. You could also greatly decrease the number of properties you will have to report.