After losing many, many auctions due to Auction Sniper failing to place the bids in time... I use 30 seconds REGARDLESS of what time of the day or night it ends. This was the general concensious I gathered when I went into the support section complaining about it. And (knock on wood) since I have done this, it hasnt failed me yet!!
You get a free snipe credit or two when the snipe fails, but it isnt any consolation when you would have much rather had the item!!