I started in early December with 16:8 and after 2-3 weeks was able to do some 18:6’s and have gone on to throw some 20:4’s in there too. Your body will adjust. I will say, if you’re super hungry, why not change your eating window to 11:30-7:30 today and move it if need be? I’m flexible with my windows but I always ensure I get 16hrs minimum fasts.
It’s always hard at first… Hang in there.. you will know when it gets easier, you can move to a stricter regime.
I did 16:8 for 2 days and found it easy, so moved to 20:4. I can push 21 as well but get light headed so sticking with this.
In short, if it’s getting easy , bump it up. On other hand, if you are seeing benefits with your regime and you are pleased, no reason to go hard. You can always mix up longer fasts here and there.
Good luck!!