To when exactly one should be waiting to get a response about a PR?

Normally you should a comment, why you are proposing a PR and what do you want to achieve.

I tried this out now, but I am not in favour of this PR: there is no contrast anymore and people with difficulties to distinguish similar colors would have serious problems Also, the header should have separators.

Well basically, the page at start.fedoraproject.org looks like a classic page from 1998. Comparing to other start pages today like Google's or DuckDuckGo. This commit fixes the problem by making it more elegant. Since the links are are wrapped around the text only, there's no significant reason for using the border in between.

For the Colors, I think it can easily be contrasted since it's not a pure white, but grey. If needed, a darker mix of white can be used.

Since this PR looks like more of a "choice" or "personal preference" which may by treated differently by people. is there a mailing list to ask people's opinion or maybe if they can also offer their own re-design for that page? Cause it really hurts me too as a user.

I think you mean your redesign when you speak about personal preference. Things don't work like that in a community, just by saying "your stuff is aweful, I have what you need, so apply it".
I told you already the things which do not work in your change, now you can try to improve that or continue with what I said above.
We also have Fedora colors and layouts of other Fedora websites we want to keep in this website.

Please also read our docs, then you would have seen the link to our Mailing List:
https://lists.fedoraproject.org/archives/list/websites@lists.fedoraproject.org/

Thanks.