The Default Expiration (In Days) found under Settings > General > Quote Settings will be set to 30 days by default. If you don't wish to have an expiration on Quotes, set this field to 0.

When creating a Quote, you can adjust the Expiry date on a Quote-by-Quote basis by adjusting the date accordingly:

When editing a Quote, the Expiry date can be further adjusted on a case-by-case basis depending on the deal being worked:

If the Default Expiration (In Days) is set to 0, the Quote Expiry field will not be present on Quote Create or Edit.