How would I force Mac App Store to save my password indefinitely, rather than for 15 minutes? I'd like to either force App Store to save my password, or have it automatically entered (just for the Mac App Store password prompt) when it asks for my password. I want this because "require after 15 minutes" is far too limiting, since it's a pain to get my password. How can I force App Store to extend the duration of saving to indefinite, despite that it's designed to save the password for 15 minutes?

Note that I know this isn't the case for free apps. I want to purchase apps without having to enter my password. Is there a way to do this?
This is by design. It is meant to ensure that you indeed are verifying the purchase, especially since it involves money.

If you find entering password painful, consider setting up Touch ID (only applicable on MacBooks equipped with Touch ID) to authorize App Store purchases. 

To configure, go to System Preferences → Touch ID and enable for iTunes &amp; App Store. Also, go through this Apple Support document:


Use Touch ID on your Mac


In case your Mac doesn't have Touch ID hardware, consider using a password manager. A password manager lets you create, save and manage strong passwords for app and websites, and enter it for you automatically. This way you get both convenience without compromising on security.

There are many good third party password manager apps for Mac to securely store your password, so you can copy and paste it if for some reason you don't trust the keychain from Apple.

Search the Web for password manager for Mac.