How can I find out how long someone was on a particular website from my browser history? And can I find our a specific date and time? Not just generalized by weeks.

Unfortunately, no on the duration.

Only events can be recorded, not durations.

Remaining at the same web page is not something that is noticeable, especially if the user uses the back button to leave a site. You would have to know when the next page was loaded, IF one was loaded immediately after.

Also, be aware that not all porn downloads are intentional. I once was attacked by a hijacked website that downloaded hundreds of porn popups, while searching for information on the Three Stooges.