How much time you have to retract your vote?
I believe the magic amount of time is five (5) minutes.
Why it is different on my own posts?
I'm not sure how you'd be voting on your own posts?
Unless you are suggesting your posts are somehow different and people were able to remove votes from just your post? I can assure you if you think that is the issue, you are mistaken. Nobody has any different permissions (I believe even mods don't have the power to change their votes except the same as you or I do). Your logon is no more special than mine is.
Why I can't retract it after the specified time period has elapsed?
I believe the reason you cannot retract it is because of how it affects those who you have voted on. IF they have edited their posts, you can retract your vote. I think the general idea of five minutes is given in case you change your mind. Once the time is up, it's done. I don't know if there is any time limit on retracting your vote after the post has been edited. I would suspect if there is a limit, that limit is fairly long (at least 24 hours) to give those who voted a chance to see the change and retract if they feel the change has in any way improved (for retracting a down vote) or killed a post (retracting an up vote).