[iojam]

NitroXploit,

for the large sites it may be required to either 1) increase max_execution_time setting value in php.ini or 2) run sitemap generator from the command line (setup a system cron job).
The new version of vBSEO Sitemap Generator (1.7) will support the "resume generation" feature that will allow you to continue the process from the point where it was interrupted.

siriuxs,

it may take more or less time to be indexed depending on many factors.
Here is an excerpt from the Google's "About Sitemaps" page:

we cannot make any predictions or guarantees about when or if your URLs will be crawled or added to our index. Over time, we expect both coverage and time-to-index to improve as we refine our processes and better understand webmasters' needs.