natural yeast, how long should I ferment?

This is a tough question, as there are far too many variables.

I experimented with my first batch of cider last fall. The apples were my from my neighbors yard (yellow delicious?) My sanitizing routine was very lax (hot water only, nothing else). Brown sugar and clove honey was added. Nothing measured. I bottled in Octoberish.

I didn't add any yeast. I wanted to try it naturally. The fermentation was slow. At peak, I think I had maybe one bubble per 12 seconds. After a month, things slowed down to about a bubble a minute. Around February, one bottle completely stopped bubbling (slight negative pressure). So I bottled it and consumed. It wasn't bad, but a little sweet. It's June now, and the other bottle still shows slight activity. It's probably one bubble every two to three minutes.

My question is, how long should I let it go? As long as I have activity, even if it's slight, should I just let it go?

Airlock activity (or lacke thereof) is not an adequate indicator of fermentation activity (or its cessation).

Trust your gravity. Plain apple juice should drop to SG 1.000 or below. With the honey (which can contain some unfermentables), you might see a final reading a little higher than that.

get a hydrometer, or rack and let it sit another 6 months.

And, can you read through it? I know, it doesn't really tell you much, but hey, at least I can read... I'm proud of that!

;-)