Usually about 2-3 minutes on average. But obviously, some moves are made in 10 seconds. Others might take 15 minutes.
How much time is usually spent per move with classical time controls?
As already mentioned, you can calculate an average time per move. For example, if the time control is 40 moves in 90 minutes, then you get 90 / 40 min = 2,25 min or 2 min 15 sec per move. And if you have an increment, you just add that to the average move time. Sometimes you get extra time after move 40 or 60, and you can redo the calculations after you get the extra time.
But the average move time is not the same as the time you should take for each move. You can play the opening moves quickly, as long as your are still in your "book". Also forcing moves can be played quickly, but make sure the move is actually forced. Then, for critical positions, where there are lots of lines to calculate, you can and should spend much more time to get it right. What is a critical position, is a topic for another discussion.
I tend to use a more time in the early middlegame after I have played my last "book" move. It's not rare to spend 5-8 min per move, when the average time is closer to 2 mins per move. I rather play with a good position with a little less time at a later stage, than with a bad position with lots of time. Because most of us play blitz games anyway, it's comforting to know, you can always speed up if necessary.

Yep 3 minutes per move. In the past the time control was 2h/40 moves, which is exactly 3 min/move. Besides that after 40 moves in general the endgame has been reached and you can play most moves faster.
I wanna figure out how much time to budget for each move for OTB classical tournaments. I'm not experienced in playing with classical time controls, and I'm worried I'll spend too much or too little time per move. Outside of opening prep, how much time do players usually spend per move? Obviously, you can't answer this question exactly since it depends on stuff like how much opening theory you know and how complicated a position is, but a ballpark estimation would be great.