I don't believe that time, i.e. number of hours, is the key determinant of whether we work too much or not, though it is relevant. The context in which we work is much more important.
When we work at what we love doing then work can be exhilirating, though tiring. In that case the issue becomes our physical limitations (how much energy we have at any point in time) and the other aspects of our lives that expect or need time from us such as families, friends etc.
We can be being underused, i.e. not working enough, and find that very stressful and exhausting.
Knowing more about who we are and how we work best helps establish patterns of work that mean 'too much' is determined by how we feel rather than how many hours we work.