How about suggesting that she set up a little experiment. Maybe she'd like to get a little notebook and for a day or two she could record how much time she spends on various activities. Then, maybe she'd like to analyze that data and make a plan for her ideal day. If she's looking for mental stimulation some of this data could be put into a spreadsheet.
The purpose of this activity is that it would take you out of the position of being the one to be responsible or to be blamed when she runs out of hours in the day. And, it would also help her see that she is not stuck. She can observe what is happening and make a different choice.
All that said, it never hurts to offer some empathy and understanding. Many times I've wished there are more hours in the day to read and it is a disappointment when I'm tired and I have to let a good book go. Sometimes at the end of the day I feel regret when I realize that I could have spent my time differently.
