As with all things, there are multiple right and multiple wrong answers to this question. Which is the right answer depends on multiple factors like the local customs, type of restaurant, crowds, and the composition/behavior of your party.

In general, I find it acceptable to do a little lingering at a restaurant. But how much lingering varies wildly.

At a buffet, people in my party generally will continue to eat until we're ready to leave. However, the waitstaff at most of the buffets I frequent stop bringing drink refills or even paying attention to the table once the bill is paid, which encourages lingerers to leave. We generally linger about a half-hour after the bill is paid, though we have lingered as long as an hour.

At a chain restaurant, we tend to shorten our lingering a bit, as we are aware of those waiting for a table out front. The longer the wait, the less we linger; we never rush our meal, but we feel that it's courteous to others to avoid monopolizing the table once we're finished the meal. If we feel the need to continue socializing after dinner, sometimes we'll go somewhere else.

I patronize true fine dining establishments very rarely, but when I do, I operate pretty much the same way I do at a chain restaurant - a little lingering is okay, so long as I am not causing others to have an excessive wait for a table.

Another factor to consider, though, is behavior. My circle of friends can be pretty boisterous, which is perfectly acceptable in a noisy buffet, and not too out of line in a chain restaurant like Red Lobster or Longhorn. But the nicer the place, the quieter it generally is, and the more out of line my party's boisterous nature becomes. As such, at the nicer places, those in the party who are cognizant of such things tend to shoo the group out once the bill is paid, to prevent the boisterousness from becoming excessive and disturbing other patrons. Fortunately, none in my group are heavy drinkers, so we don't have to deal with rowdy drunks. Simply saying, "Everybody ready?" after the bills are paid is typically enough to motivate the exodus.