A Quote is dated as at the day it is sent or marked as sent. This is when the Quote becomes 'active' and what sets the Quote's expiry date.
To set your default Quote expiry for all your Quotes, go to Settings > Quote Defaults and enter the number of days that you want your Quotes to remain valid.
At times you may need to change the expiry date for a particular Quote. You can do this while editing the Quote by clicking on the 'Valid Until:' date link.
Quote dates will re-set if you edit a Quote after it has expired.
A customer can view an expired Quote but they may no longer accept it, therefore if a Quote Author edits and resends an expired Quote, the expiry date will automatically be reset so that the customer is able to accept it.