How much time Google Scholar take (generally) to index an arXiv document?
It usually takes a few days according to this answer, which is in-line with my experience (I don't really monitor it though). In some cases, I think it can take a couple of weeks for Crossref which may give an idea of how long it could take for Google Scholar.
If you want to, you can manually add the article to your Google Scholar profile. However, I'm not sure how quickly that affects indexing.