First book, first publisher, 6 months.
Other books? So far, not so lucky.

However, we're nowhere close to despair territory, most of my others have been rejected once or twice and are onto the next. One she just got so it could have that golden "first try - sold" glow on it, too. We'll see.
There really is no average, though. I've seen 6 months, I've seen a year, I've seen longer.
I accepted long ago that this is a hurry up and wait business, and once the property is out of my hands, I forget about it. This makes my follow up on shorts none too swift, but it also means I'm busy writing the next things and submitting them.

It works for me.