I've experienced a few times in the last few days where I couldn't edit my own post after 5 minutes. I understand the alleged reason of it's existence, but I necessarily don't agree with it. In fact, while participating in dozens and dozens of other forums and viewing hundreds more, I don't recall seeing anything similar.
The most use of any limitation I see is after someone posts a reply, then the post can't be edited. But even so, I still see many forums even after someone posts, the original poster can still edit their own post.
I suggest that there isn't a time frame involved in editing a post. It doesn't make sense to me. I do suggest that it's better once someone has replied to the post, then there can be a no-edit involved. At other places I've made several posts where I go back 10 or 20 minutes later, or even more and see no replies, then think of something different I want to subtract or add to the post or make a grammar or spelling edit, then have the opportunity to make those changes.
Something to consider.
Thanks,
Matthias