Not regular but whenever I feel like I need one. I optimize for most productive stretches of time. Design is part grind and part creative. You can force the grind but you can’t force creative. Ideas come at random times too and you sometimes do most of the work in your mind right after waking up or doing some totally random thing. 

I guess it helps to enjoy your work or else id be burned out more. It’s nice being able to say that I have fun working so it never feels like work.