It kind of depends on how much time and mental bandwidth I have for each illustration, but since I switched from working on a comic every day to doing one illustration a week for my novel, I've been able to put a lot more of myself into each drawing, and really stretch to learn new skills (with mixed results lol). I've been a lot happier with my art as a result, though working on the comic did help to improve my skills rapidly in the beginning.
I have another WIP that I've poured EVEN MORE time into, and hope to actually finish one of these days (luckily I've learned the skills to do that since I last worked on it), so really given more free time, I could push my illustrations even further... there's just never enough time, unfortunately.
Here's the WIP in question: