I would recommend that users follow the conventions of each specific forum.
For example, several that I’ve participated in use “ETA:…” or “edited:…” on a reply to indicate additions or alterations, typically in an effort at transparency (as edits are hidden from most users).
One group in the past tended to italicize edits that weren’t mere typos or autocorrect fails.
Another group didn’t seem to care.
My thought about how long is reasonable after posting is that just because no one has replied does not indicate that no one has read the post, so keeping edits as transparent as possible is simply courteous.
In general, if you still have time to edit, go for it! 
