how often are triple stopped violins used in orchestral music?

Depends on the era. If you say orchestral music, I'm assuming you mean a modern orchestral size? If that be the case, most of the time triple stops would be divided upon the players. It would be extremely difficult to have all the violins do a triple stop in sync. If it's a Baroque group and use Baroque instruments (which, it would probably be a chamber group), their bows are shaped different than the modern bows, so triple stops are more doable.

I wouldn't say fairly, as most of it is divided between the parts, like outside player plays top notes and inside players play the bottom notes.

In romantic music they are commonly used, however, instead of triple stopping, conductors prefer violinists to divide to improve intonation.

Triple stops are usually played as triple stops in big endings, but a loud sound needs to be produced :)

Fairly often. I'd say most concerts that aren't strictly Baroque music include triple stops somewhere.