I know there is a rule for geocaching that says x # of feet between caches. Is there such a rule for letterboxing? I saw “poaching” in the glossary, but there was no distance given.

With letterboxing you will need to use your judgement. There is a train museum in Ogden that had three letterboxes (by three letterboxers) in the trains, about 300 feet long and about 100 feet wide. One is missing, one I have at home (and I need to put back) and one is still there.

You can inadvertently poach a puzzle because you don't know where it is at, but there is no set distance.

As O_K said, yes it's possible to accidentally plant quite near another letterbox.
Because of restrictions, you can't always see all of the boxes planted in a given area.
If you are planting near someone else's box, just don't reference their box or use their clues.
We have two boxes in out area that are a mere five feet apart. Two different planters, two different sets of clues and you'd never know that other box was there because each is so well hidden.
My best guess is that the second boxer was unaware of the first boxer's plant.
Another pair is on opposite sides of one tree. Two different boxers with very different clues.
It can happen.

Another pair is on opposite sides of one tree. Two different boxers with very different clues.

Must be an awesome tree!

We have two boxes in out area that are a mere five feet apart. Two different planters, two different sets of clues and you'd never know that other box was there because each is so well hidden. [...] Another pair is on opposite sides of one tree. Two different boxers with very different clues. It can happen.

But is either poaching?

If you are planting near someone else's box, just don't reference their box or use their clues.

This seems to indicate that these (above) boxes aren't poaching. Don't try to plant so close, but if you do (unintentionally) it is apparently okay.

Referencing another box reminds me of a box that I planted that does reference another box series (of three boxes). Because I wanted to reference that other series I asked the planter of that series if I could reference their series, and the reference was okay. I planted among their series but at least 100 feet from the nearest of that series. The box was related (in theme) which is why I planted it there.

three letterboxes (by three letterboxers) in the trains, about 300 feet long and about 100 feet wide. One is missing, one I have at home (and I need to put back) and one is still there.

Wow! Those are some BIG letterboxes! Tough to hide. No wonder one went missing...

Wow! Those are some BIG letterboxes! Tough to hide. No wonder one went missing...

If you think that they're difficult to hide, you should come and try picking them up to stamp your book. ;-)

I said

Another pair is on opposite sides of one tree. Two different boxers with very different clues.

you said

Must be an awesome tree!

three foot diameter.
funny thing, I remembered being there before so knew about the other box.
we were stopping only because I remembered, and we accidentally found the other, other box.