Linkvalidator is a system extension.
If you are using Composer, you can install it like any other (core) extension requiring the package typo3/cms-linkvalidator.
If you are not using Composer, you may have to activate Linkvalidator in the Extension Manager.
Linkvalidator uses the HTTP request library shipped with TYPO3. Please have a look in the Global Configuration, particularly at the HTTP settings.
There, you may define a default timeout. Generally, it is recommended to always specify timeouts when working with Linkvalidator.