A frequent discussion over on the Composting forum here. They even have several FAQs about it. But the simple answer is it is done when it no longer looks like the ingredients that went into it, when the various ingredients are no longer recognizable.

As to when you quit adding things to it - that all depends on the method you are using. A container or bin - when it is full. An un-contained pile - when it is of a manageable size and then you start a new pile with the ingredients and let the first one 'cook'.

Dave

Composting forum

Composting FAQs

Unfortunately, if you grind up your material first (I use a mower), it's going to be a long time before it doesn't look like the ground up stuff that went into it, and nothing is ever really recognizable. It kind of always looks like wet chopped up stuff. The best way to tell is with a thermometer. When the internal heat in the pile starts to decline, that's when it is ready. If you don't have a thermometer, you can stick your hand inside and see if it's starting to feel cooler.

Ideally this is after the pile is stirred a few times. That is, the outer layer of a pile isn't going to compost. Only the inner part is. So if it starts to cool in the middle and you haven't stirred it, once you stir it, it'll probably heat up again. Of course, check the temp a few days after you've stirred it.

I have a small pile right now (mostly leaves) that was at 120F for a month or so, after being stirred a few times. It's now dropping to 100F. I think it's just about done.

I should add that the high temperatures that develop in the process of composting are one reason why we don't bury a lot of uncomposted organics in just-to-be-planted soil. Because it's gonna get hot down there! In fact burying organics in a bed that is being solarized increases the temperature beyond what the sun can do, and makes for an effective and sustainable soil fumigation strategy. That's called "biosolarization".

Digdirt2 - Thanks for the links. Lots of good info.

Daninthedirt - Thanks for your input too. I think mine must be getting close. I've stirred it as best I can a couple times and the temperature hasn't risen much the last time. I'll hope to stir it again taking the temperature before I do and then a day or so after. If it's stable, I guess that means I can start using it!