How common? Very common +
Every Unforgivable curse has been used multiple times in the books, so mostly over the course of seven years, by Death Eaters, Dark Wizards, and even the protagonists. The Harry Potter wiki has a long list for each of the three curses:
- Gellert Grindlewald: This Dark Wizard predated Voldemort by decades, and was already familiar with the Cruciatus curse when he was barely out of school, when he used it against Aberforth Dumbledore.
- Dolores Umbridge, while not a Death Eater, was arguably a Dark Witch. She showed willingness to employ the Cruciatus Curse against Harry in order to "loosen his tongue."
- Bellatrix Lestrange: She employed the Cruciatus curse against Hermione and Neville, along with possibly helping drive Frank and Alice Longbottom insane through prolonged torture.
- Rabastan Lestrange and Antonin Dolohov also participated in the torture of the Longbottoms.
- Bartemius Crouch Jr. also helped torture the Longbottoms, along with bewitching Viktor Krum into using the Cruciatus curse on Cedric Diggory.
- Draco Malfoy: He tortured Rowle as punishment for failing to capture Harry Potter.
- Many other Death Eaters used this curse on various occasions, including Alecto and Amycus Carrow.
- Harry Potter employed the Cruciatus curse against Amycus Carrow. He also attempted to use it against Bellatrix Lestrange.
This one is amusing, because numerous sympathetic characters have employed it, albeit usually under exigent circumstances:
- Harry Potter used it on the goblin Bogrod and the Death Eater Travers in order to break into Gringotts.
- Minerva McGonagall employed it against Amycus Carrow.
Various Death Eaters has of course used this curse, among them Yaxley and Wormtail.
Bartemius Crouch Sr. also used the Imperius Curse, in order to keep his son under control.
This is the go-to offensive spell for the Death Eaters. It has certainly been used by Peter Pettigrew, Bellatrix Lestrange, Thorfinn Rowle, Severus Snape, Vincent Crabbe, Bartemius Crouch, and Antonin Dolohov.
We can, however, assume that Avada Kedavra is most likely the curse meant when "a jet of green light" flies over Harry's head or something similar. In this case, the Death Eaters must be throwing Killing Curses around like water.
Needless to say, Lord Voldemort has used all three Unforgivable Curses multiple times.
Various Aurors also likely employed these curses under Bartemius Crouch Sr.
We can also suppose that these curses were at least moderately common before Voldemort's time, since they were made unforgivable in 1717.
At least among Dark Wizards
Do people practice them?
I haven't seen anything in canon indicating whether ordinary wizards, or even Aurors or Order members, practice these spells. I am aware of one particular situation where people did practice the Unforgivable Curses, and not in front of a mirror. Perhaps at Durmstrang they did something closer to what you imagine:
“Amycus, the bloke, he teaches what used to be Defense Against the
Dark Arts, except now it’s just Dark Arts. We’re supposed to practice
the Cruciatus Curse on people who’ve earned detentions—”
—Neville Longbottom, Harry Potter and the Deathly Hallows