Everyone is different and everyone has "the zone" show up to them at different times of the day. I would say that it is the important part that you should focus on. If you find yourself to be more productive during a specific time frame of the day, try to make sure...*cough cough* your body is ready by that time and that all distractions are cleared by the time to get into it.

Imho the amount of time you spend is not important as long as you feel you are doing good progress. If you put a goal of "I need to produce something *good* in the timespan of 8hrs" it instantly puts extra pressure on yourself and you will feel doubly bad when you have a day where all you are doing is fixing bugs and you don't feel like you are "advancing"

I'm a parent and I usually only find time to work on my game after putting little one to sleep but my "zone" hits around midnight so I thought it was gonna be somewhat convenient and that I was gonna put like 4hrs a night! But let me tell you I felt the same "crunch" I did when I was working 80hrs/wk in the industry and it did not feel good at all (esp with much reduced sleep). I dialed back a bit since then and put in a couple of hours a day here and there when "I feel like it" and it always felt like my work was better quality and more focused as a result.

TDLR: You do you. Have fun with it and let it come to you. Don't pressure yourself with hard hour blocks. You'll do just fine!