For how long, after the first message by the mediator, if the client does not reply to the dispute, the dispute is won by the Freelancer?
or What is the scenario if the client is unresponsive in the dispute?
If one side never responds, the other side automatically wins.
The length of time that a dispute moderator waits is five business days.
So if you ever are involved in an Upwork dispute and the client doesn't respond, that is a good thing for you.
As a practical matter... If a freelancer has finished a fixed-price contract task, and the client isn't releasing the funds and then the freelancer files a dispute...
If the client doesn't respond to the dispute notifications/requests for response... 99% of the time it is because the client knows full well that she is in the wrong and she is too embarrassed to respond. So she will just let it go and the freelancer will receive the funds.
What is she going to say?
"Yes, I know that I hired him to work for me, and he did what I asked. But I saw a button I could click to block him from getting paid, so I thought: What the heck? I'll click that button and maybe I won't have to pay him."
The mediator gives everyone three business days to respond, but don't get excited if they don't respond in the given time. The mediator always gives them extra time. It's happened to me twice.