The most likely reason is that Google has temporarily blocked you for importing too many items too quickly. Try to click the “Cite” button under and item, then click the “BibTeX” link and see if you get a Captcha to complete. If it just blocks you, you will have to wait until Google unclocks you (you can try to delete your cookies to speed that up).

See here for tips on importing large numbers of items from Google Scholar https://www.zotero.org/support/getting_stuff_into_your_library#large-scale_imports_from_databases

Hard to say. Not usually more than a few hours. Try to delete your cookies and clear your cache to speed it along.