Miles don't really mean anything, you can coast a lot or you can pedal hard. You could have lots of hills or none at all. Mileage is not a good indicator, but it's better than nothing if you need something.
I did 100 flat miles on Sunday at a slow speed with another rider. It was easy. In the mountains I might not be able to even go 20 miles. With a good headwind it's a lot more work. At maximum effort, I can't go far either.

The real thing is intensity and duration. Say something like

1.5 hours Monday medium effort. That kind of thing.

The stationary bike is good exercise, but not equal to miles on the outside bike. They vary all over the place. Too many variables.