I would suggest setting aside a few hours on one of your weekend days to dedicate towards a project of your choice. How many hours is up to you and your schedule.
Don't set aside anymore time than that. Setting aside more time will only make you feel demotivated if you don't make it, but if you DO have extra time you want to work on your project, you can do so and feel good about it.
Don't forget there's a lot of downtime when your mind isn't busy (commute to work, lunch breaks, doing chores, etc) when you can still be thinking about and planning your project. The block of time is mainly to give you a dedicated period in the week that you can put towards coding it.