I always go for 8 seconds. My reasoning is this:
1. AS will adjust any lead time so that there is a pretty good chance of the snipe arriving on time depending on the 'conditions' towards the end of the auction
2. The chances of being beaten by a manual snipper reacting to your snipe in the last minute are almost nil.
3. If another snipe comes in after yours then it will only win if it was higher than your highest amount and it would have won anyway whether it arrived before or after you. Obviously, if it was for the exact same amount then you'd want yours to arrive first so a small lead time would be a disadvantage.
Hence 8 seconds is a good default. Not too small, not too big. Works for me...