I believe a time limit was set on editing in one of the more recent updates, though I can't personally attest to any behavior it may be having now since I can edit anything at any time. Is what you're seeing inconsistent with that?
I'm not certain about any time limit per se, but the edit button will disappear (and always has) when another post is added to the thread. I'm sure that this is intended behavior.
The issue at hand: The edit button should appear immediately along with a new reply. It did up until a fairly short time ago, but does not now. It *will* appear eventually, taking anywhere from a few seconds to many minutes to do so. (Much more often the latter anymore.) If someone adds a reply behind, before it appears, one cannot edit at all.
This is tied in, I believe, to another recent behavior change- the amount of time it takes for a reply to be added from the thread onto the front page. When I first started visiting these forums, it would take quite some time for this to happen. When the forums were updated, things were sped up considerably. Unfortunately, things seem to have reverted back to the pre-update times. This happened ~the same time as the edit button problem cropped up.
So, in a nutshell (from personal observations): The edit button will only appear once the reply makes it onto the front page. This can take some time- and if a member doesn't have the patience or persistence to wait it out, then it will seem as if it had never shown up at all. And, if someone comes along and replies before this has had time to happen, the button will never show up, and the reply in question can't be edited at all.
EDIT: The edit button appeared immediately as my reply showed up in this thread. Maybe the problem has been fixed. (crosses fingers)
EDIT#2: This reply showed up on the front page almost immediately.