It shouldn't be like that. Most internships are three months and typically in the summer between academic years for undergraduate or graduate students. They are paid internships, but much less than a permanent position (enough to cover base rent and travel logistics.)
The stress should not be too high as most companies have policies that interns can not be assigned to essential work activities (and this might be the law in some US states.) It's typically 'educational' in some form for them as it lets them see how the sausage is made.
