The reward/cost of practice time

I started playing at age 55 and have been at it for over three years. I practice every day, usually 2-3 hours. My teacher, who has a PhD in piano performance, talks about eight hours per day of practice while getting her degree. That's bonkers to me. My brain is usually pretty fried at the two hour mark.

I think we can all agree that daily practice is much better than cram sessions. But how much can one usefully practice at a time? My feeling is that the first hour is the most valuable. I break my practice up into several sessions of 40-60min., but the first one of the day is usually the best. And the one after lunch isn't bad.

What do you do?