Sellers have a wide range of PunchOut quote expiration periods. These periods could be just a few days for items with volatile pricing to several months for items with stable pricing. Some sellers have no quote expiration policy. If you have an expiration policy, the quote period should ideally be declared in the contract with your customer. The contract would have language such as, "quotes provided by our PunchOut site are valid for 60 days from the time they are issued."
After a successful PunchOut session, it might take the buying organization several days or weeks to approve the requisition due to vacations by approving managers or other roadblocks. In short, sellers should not ship items or perform services until they receive a PO. However, most sellers will want quotes to expire after a certain amount of time. This should be explained to the buying organization through text on the PunchOut site, through the contract, or both.
If you receive a PO after a quote expires, send an order confirmation that either changes the line-item price (if allowed by the buying organization) or rejects the PO with comments that explain why. Alternatively, you can accept the PO as-is and change the price in the invoice (if allowed by the buying organization). You should ask your customer which method is preferred.
Expired quotes can be useful to sellers. You can investigate why the selected items weren't actually purchased to make changes to your PunchOut site for better customer satisfaction. You might want to provide a report of expired quotes for this reason.
Hope this answers your question.