The edit timeframe is set for 15 minutes. This is in the interest of keeping everyone honest about what they write. Spelling mistakes are no big deal - and there is a preview screen option to find those, anyway.
If you have reconsidered a postion you took in another post, just make a new one.
"In case you ever wondered what it's like to be knocked out, it's like waking up from a nightmare only to discover it wasn't a dream." -Forrest Griffin