Sustained attention to walking in a forearm support frame is usually about 30 minutes. Once in the morning and once in the afternoon is a common schedule that can prevent yelling.
If their attention can be captured to do something while sitting, they can usually sit in a chair for about 30 minutes without sliding out.
The need to be in an upright position is about 2 or 3 hours a day to prevent yelling. If the person has become a chronic yeller, more sessions per day may be required to break the habit. A regular schedule for being up during the day can normalize circadian rhythms so that the person sleeps through the night without waking up to yell.