I find in one-on-one training, that even de-conditioned older adults above 70 years of age benefit from an hour of training time. Not so much because they are exercising all that time but because they are learning the exercises, and that often takes a little more time. I can imagine that this is even more true in a group where you may need to divide your attention.
I have only one client who trains for 30 minutes because of a very special medical condition.
I would also be careful with the term ‘senior’. At this point in time, seniors come in so many different levels of abilities that you can no longer categorize them as ‘that population’. Being almost 58 myself, I speak from personal experience.
Good luck with your class.