Quotations
Quote Validation
To modify how long a quote will be valid for simply change it within the quote on the left column 'Valid For'. However if you want to change the default value to all of the quotes you create you will need to change this in the settings.
The default quote valid time can be found under 'Settings' > 'Company Details + Setup' > 'Module Setup' > 'Quotes' > 'Default Quote Valid for (Days)'