For flat work 45 minutes is enough for me and 20 to 30 minutes if we jump. But as everybody said, it depends of the horse's condition and of the weather (very hot, a lot of sun, very cold, ice on the ground...).
In clubs, we ride about 1h but I think if I had my own horse I would ride less as I could work more efficiently alone or just with a coach. I think it would depend of how the training is going, if we encounter difficulties or not. And it is important to adapt the work and time of training to the age and health of the horse of course!
I adapt everything to the horse I ride. Some feel more comfortable with a quick warming up, some don't. And I ride depending on the horse's state of the day. If I feel him tired, I will be satisfied with little. So a training can last 20 minutes or 1 hour, it really depends of what we do (dressage, flat work, jumping, trotting...) and how the horse feels.
Actually it depends of the shape and age of the horse! I am used to ride an hour but my horse is quite young and in good shape so I know it is not too intense for him. We walk for 10 minutes, then trot, then canter and then start to really work so he can warm-up. When we jump, it is only 30 minutes and then we do some flat work or go for a walk as it is more physical than 1 hour of flat work.