Yes, if you think the guy does not have to wait after he says “yes”, then we are not talking to each other.
If the guy is free to go, I don’t see why the girl has to answer ever. Why should she answer yes or no? If she ever decides to go out with him, they will call him up. (Similar to what I think about when a shadchan calls a guy first.)
As it happens, the guy is not allowed to date someone else after saying yes to someone. That is why we are discussing when the “yes” expires.