How do I know when my sauce is reduced enough?
The whole thing should've been done with the lid off.  Any time you're reducing a sauce, you want the steam (moisture) to escape.

As for 'how thick', the standard test is 'coats the back of a spoon'.  If you stir with a spoon, you should be able to lift the spoon out vertically, and the sauce doesn't immediately drip off of it.

This test also lets you judge just how thick the sauce is, as you can look at how much is coating the spoon.  If it's too opaque to be sure, you can run your finger across the back, and judge the clean strip vs. the coated part.