How many minutes of cardio do you think I need to replace a 20 minutes hiit traditional session on a certain day that I suddenly want?
(And I mean with the cardio of skipping rope at 70-85% of my max heartrate)
You can't effectively replace one or the other on a certain day suddenly because you feel like it. HIIT works different metabolic pathways than general cardio. You should have chosen one or the other depending on your fitness goals, and to replace one randomly because you feel like it means you're not following your training program.
If you decide to skip your HIIT training one day, you can do cardio in its place, but there isn't a certain number of minutes that will give you an equivalent stress or adaptation.