I think 25-30 minutes is a good base line. Personally I like to focus on quality than quantity when I'm lunging the younger horses. It would also depends on how much I have done the day before, how much I'm planing to do the next day and what I'm trying to achieve I.e. Am I lunging to get them fit or to improve their balance and top line?
I normally do 25-30 minutes. For young horses I spin them on the lunge for 5 mins on each rein before getting on if they are fresh. As they get older, I lunge them less frequently. My 9 year old PSG mare in never lunged - she is far too fresh!
Depends when the last time you rode was and how much energy they have. Sometimes you’ll play it by ear if your horse has lots of energy you’ll want them to buck and pig root a little bit to get out some energy and if hey don’t do this in the first 20 mins or so then you might make the decision to stop or to keep going. It’s really up to you
I usually ride for an hour so lunge for about 30 minutes. Its physically harder for the horse to go in circles so I do half of when we ride. But it really depends for me because I sometimes want to do a small excerxise.