Making old Quotes new again
Overview
It's good practice to always include an expiration date with your estimates. Even when business is bad you may not want to honor prices you quoted five years ago. That's one aspect of managing an estimate, adjustable either in the default memo clip, or directly in each estimate, case by case.
The other part, the one you set in Estimate Preferences, is how long those old quotes should be available for inclusion in new estimates. The default is 30 days. You could extend that, but a better solution is to always open old quotes for editing and then save the update. Morning Flight will automatically reprice the quote and validate its entry ticket for another 30 days.