You can set a global timeout in:

Settings → CI/CD → General pipelines → Expand → Timeout

The default value is '1h'.

As of version 12.3, you can set a timeout per stage in your CI .yml file using timeout:

build:
  script: build.sh
  timeout: 3 hours 30 minutes

test:
  script: rspec
  timeout: 3h 30m