From AS Faq ...
9. When can I add, modify, or cancel a snipe?
You can add, modify, or cancel a snipe as long as there is at least 3 minutes left until the auction ends. When the auction is about to end, you may notice that a lock will replace the Cancel and Modify links. In the event that you have not refreshed the page, and the Cancel and Modify links still exist for a snipe that is ending in less than 3 minutes, any changes made will be disregarded. For reliability and performance, our bidding system prepares snipes three minutes in advance and so you must make your changes before this time.
Mother Mary Says, '
HONESTLY! you're just ruint!'