When to clear Redirections?

Hello,

Thank you for contacting Rank Math with your redirection questions. I will do my best to answer them

1-
I am noticing that when I change the category of a post, it creates a redirection. I guess this used to not happen earlier and it used to adjust by itself (I mean I did not see the entries in the redirection section)

This is how the redirections should behave.

2-
We create redirection (mostly 301) to prevent duplicate posts, etc. How long does it take for search engines to acknowledge the 301 request. In other words, when can we clear the re-directions?

The time it takes for Google To crawl and reindex your pages varies but can be up to two weeks, depending on the amount of content and Google’s crawl budget.

3-
I am unable to create regex
e.g. if there are multiple URLs beginning with the same pattern (/download/wallpaper/????) and I want to redirect all to a different landing URL (/picture/gallery) (n.b. I have read the regex section and tried but failed)

you may use the following .htaccess redirect:

RewriteEngine On
RewriteRule ^download/wallpaper/ picture/gallery/ [R=301,L]

I hope that helps. If you have questions, ask away. We are here to help.