Hello,
1. if you selected "run in background" checkbox when starting generator, it might keep working.
2. Sitemap is fully regenerated every time.
The crawling time itself depends on the website page generation time mainly, since it crawls the site similar to search engine bots.
For instance, if it it takes 1 second to retrieve every page, then 1000 pages will be crawled in about 16 minutes.
Some of the real-world examples of big db-driven websites:
about 35,000 URLs indexed - 1h 40min total generation time
about 200,000 URLs indexed - 38hours total generation time
With "Max urls" options defined it would be much faster than that.