How long should I wait before responding to a request?

If you're able to provide what's needed in the brief then it's best to respond as quickly as possible...

Are there benefits from replying early?

The sooner you respond, the more chance you have of getting in front of the journalist before other respondents do...

I am already a customer of a paid plan. Can I register for the Starter plan?

Sure, however, being a client of a paid plan means that you will have more benefits than the Starter Plan, so we don't see any advantage in it as the benefits are not cumulative...