No. There are really no hard rules for fasting other than no caloric intake, your window can vary from day to day.
It’s probably easier to have a set eating window every day for most people. But you can change fasting periods from time to time.
And you can also extend the fasting hours to a longer time if you feel that’s good for you. Some of our fasters have chosen the 16:8 fasting plan as the baseline, and rarely fasted for 18 hours or 20 hours for some accessional days.
Just make sure that you can reach the least hours which your fasting plan requires, for example 16 hours for the 16:8 method, to keep your body in a fasted state.