In my earlier efforts at writing, I used to have a mental image of a well, an "Inspiration Well" that I monitored constantly. I could tell just going to that image whether I had the inspiration to write something or should wait.
If the well is half empty, I'll run out of ideas mid-story. If the well if full, I can draw from it liberally. If the well is overflowing, I'd better get to writing before all that wonderful inspiration was wasted.
This internal gauge seemed to work.
(There is a school of thought that you shouldn't wait for "inspiration" but that sentiment is somewhat disingenuous. Of course a writer needs to have something in the well to write--it's when you don't cultivate that well, or ignore it, that you get in trouble.)
In this latter day writing period, I haven't had to worry about it. I was so full of creative energy those first three or four years that I almost always finished what I started, sometimes at full speed.
There were an occasional story I started and then stopped--but mostly because some other story took precedence, often because I needed to fulfill a commitment. When I'd come back to the story, I'd realize it wasn't strong enough and move on.
In the last year or so, I've had three stories that I didn't finish. One about halfway done, one about a quarter way done, and one three quarters done. Now... I'd be more concerned about this if I hadn't also written "Fateplay," "Shadows Over Summer House," and "Eden's Return" in the same time period.
But after taking December off from writing, I'm finding myself without creative energy. So I went to my "Well of Inspiration" and realized it was half empty.
I have zero doubt it will fill up again. But I think it best to wait.
I've got one book completely ready and already at the publisher. One book that needs a quick rewrite. And another book that needs a bit more of a comprehensive rewrite. So I've got material in the pipeline for a while yet.
I also have my three completed "Tales of the Thirteen Principalities" novellas that I'd like to plunk out there one per month (and very near completion on a fourth) that need a bit of continuity work.
So really, I have a year's worth of material almost ready for publication.