Different timings suit different people. I like to write in great chunks, with no time-limits. When I worked full-time, I aimed for two evenings a week, and found that actually three evenings a fortnight was realistic. Those evenings, writing was THE thing, from finishing work to bedtime, giving me five hours, and I could carry on into the night if I wanted. (Dinner was leftovers or even a pot noodle.) Those three evenings left the rest of my time guilt-free and I wrote more. One of my students found her best time was the morning, away from the house, and that she worked best in small regular bits. She caught the bus to work half an hour early and sat in a coffee shop peacefully writing before each day's work.
Try different arrangements to suit yourself - different lengths of time, times of day, times per week. Sit down once a week or once a fortnight to evaluate what is and isn't working. When it's working, make a note of what's working so you can keep doing that. When it's not working, look at why. The answer is never, "Because I'm shit." Even if you were shit, you'd still need to work around your shittitude, so drill down a bit for the more interesting and more useful answer. I'm "shit" when I'm overstimulated or distracted by other demands. At first, when I went freelance, I tried freelancing in the mornings and writing in the afternoons. It didn't work: changing headspace took me too long. I tried Thursdays and Fridays, but I was "shit" - because the hectic pace I set for the first three days had worn me out. I tried Mondays and Tuesdays, but I was "shit" because the anxiety about unanswered emails distracted me. At the moment, Tuesdays and Fridays works for me - I can pace my energy and know that my other work is being taken care of. I've also learnt that I can't do more than five hours, whether that's spread from 9 to 5 or in a chunk from 1 to 6. Juggle it around for yourself and see what suits you.
When you've found your writing times, the rest of the time you are not writing. You can see friends, guilt-free. Go out, guilt-free. Read books and wallow in the bath, guilt-free. Relax after the kids' bedtime, guilt-free. You can even let the desire to write build up, hungering for your next session, so instead of miserably thinking, I should be writing..., and hating writing for making you feel guilty all the time, you're wiggling with excitement: I can't wait to write!
