You know, I used to have to write only one or two days a week, on my days off. Then I would just mark out whole days for writing and spend like six hours writing.
It's tough to find time when working 40 hours a week, but if you don't make time now, you will never get to the point where you are writing full time instead of working full time.
That said, it's recommended to start smaller and just get in the habit of writing for shorter periods a day. Say, if you can find a free hour during the day and mark that out for writing. Tell yourself that, no matter what, you will write something that day. It doesn't matter if it's good. That will come as you train your brain to write every day. If you make it part of your routine, your mind sort of cues into the fact that it needs to be creative.
The best thing is to leave off when you know what's coming next, or even in the middle of a scene. That makes it that much easier to pick up the next day.
Honestly, I tried to find just a few days a week that worked for me and every time I did that, I would lose stamina. It's easier to skip your scheduled writing time when it's not part of your daily routine.
but it's all about what ultimately works for you and what your goals are. I say set the goals be they pages, word count, hours/week, what have you) and then try out different times/days and see what helps you best meet your goals.
good luck with this! it's always tough to find that balance and make time for writing, but it's definitely worth it!