Hello Pete Lyons,
One hour is a good time frame; makes it worth the time and travel.
I call the seniors, the 50+ years group or older adult.
Every senior is at a different fitness level, just like any other age group.
I play things by ear; although, I have a class planned out.
One hour long sessions worked for us, as it is also a learning experience, not so much moving in high intensity. One hour gives us more time for individual questions and concerns, too.