Yeah most studies I've seen shared online put us at about 4-5 hours of useful work a day. This seems to be what I settle into at home when left alone too.

It can of course go higher when you have a specific goal and you're running on adrenaline or are truly enraptured by what you are working on. But a typical 8 hour day is often packed with chitchat, meetings, administrative busywork and so on to help pad out those hours.

I did work one place that stressed to us to allocate no more than 5.5 hours of work-work a day when estimating jobs due to the above, and it seemed roughly accurate.