I adjust depending on the day. I only have steady blocks of hours, say at least 4-8 hours a day. No, I am not joking I said 4 to 8.
I usually do it in the morning. I used to play solitaire on my phone , I told myself it was going to wake me up but now I choose what I mentally “eat” for breakfast.
I would like to set specific times, however this is often not possible because I have a pretty inconsistent itinerary of tasks and a continually changing work/study related schedule. I find it is often easier to start studying by incorporating my sessions into other tasks and thus ritualising them. For example: I try to watch the news regularly and often I watch the previous night’s news at some point the next day. If I realize that I have got a busy schedule for my afternoon I watch the news for breakfast and start a study session following the newscast. If I have a particularly full morning/noon schedule, I try to watch the news in the afternoon and then start to study. I find it is easier for me to start studying after watching the news because they somehow provide a link between everyday stuff and the more abstract responsibility of learning.