It really depends on the context of the work. Usually I'm getting interrupted, or I worry that I'm going to get interrupted. Then there are meetings, coffee breaks between them. All these things prevent me from being productive.

I don't think working 8 hours of productive work will get you to burnout. It's more a matter of habit and discipline and interest in the project.

In my experience, what leads to burnout is a bad working environment and/or working a lot for long period of time.