Almost any time. Plant with compost in the spring. Add it as top dressing/ mulch through the season. Plant with it again in the fall.
It's finished compost when it is crumbly, smells earthy, and you can't see any of the original ingredients in it, except in mine the occasional bits of eggshell.
I don't turn my compost at all, except 3X a year when I want to use my finished compost, I turn it over/ upside down. That is, I take the stuff off the top to be the bottom of a new pile. I keep turning it upside down like that til I get to where all the earthworms are. Then if that part isn't quite finished, I stir it around a little and poke some holes through to the bottom for aeration and it very rapidly becomes finished.