It's meaningless to count hours for development work without also talking about how much of that time is spent doing actual work, and their velocity.
Someone who works 40 hours, takes regular breaks and keeps a moderate pace, sure. 40 hours of high pace concentrated development work? In the last 17 years of managing development teams, I've had maybe 1-2 people who could sustain that for more than a week or two at the time.
If it's 20 hours of time-on-task while in flow it's a TREMENDOUS amount of work in a knowledge worker job. We should all be so lucky to be so productive every week.
A lot of people's time is filled by very low-productive work such as excessive communication overhead, or fiddling around with low-quality tooling, etc.
Some problems, to be solved, require intense work.
At 40 hours a week you will hardly feel "burned out".