Free time is subjective, take for instance following Roxy
Nabiki doesn't even have to navigate because Rroxy is leading the way
So as the walk towards a destination that Roxy knows Nabiki simply shifts some of her train of thought to something more productive than simply letting her mind go blank
She optimizes her time whether it's ten minutes or hours

I used to have a friend in collage who could do this
I once watched her solve a trig equation while making christmas cookies