Quotes remain valid for a certain length of time after they are created.
A quote that stays open beyond this period will expire, meaning that it cannot be finalised unless the Valid until date is changed.
By default, a quote will remain until the end of the calendar month in which it was created.
An alternative system-wide option is available. This option lets you set a number of calendar days for which a quote will be valid.
Administrators can change this setting from:
Settings - Company Setup - Quotations and Sales
The new validity period will apply to new quotations raised after the change is made, and will not effect existing quotations.