I reported plagiarism to a questionable journal. How long should I give them to amend?
The responses that you received from the journal show that it is not just questionable but clearly predatory and rubbish. No sane scientific publication venue would say 1) wholesale plagiarism doesn't count in the introduction and 2) it can be addressed just by adding a reference.

The journal clearly has no interest or intention toward rectifying the situation, and it's almost a pure waste of your time to interact with the journal more at this point.

At this point, there are two actions that make sense for you to do:


If it's a "pay-to-publish" open access journal, make sure it gets included in the predatory publisher list by contacting Jeffrey Beall
Report to Wikipedia, who can decide whether they care enough to pursue.


After that, let it drop: the internet is filled with vast sewers of low quality junk, both scientific and otherwise, and you've got better things to do with your time.