Sunday is the busiest time for US eBay and you should allow not less than 15 seconds for your snipes at any time of the day. If the auction is ending in the evening you might want to make that 20 seconds. And if the auction is ending on the even hour, half-hour, or quarter-hour (XX:00:00, XX:30:00, XX:15:00, or XX:45:00) it's advisable to give a 25- or 30-second lead time.
Now, I'm not saying that a lead time of 4 or 5 seconds would never work. What I am saying is that on Sundays, with each second less than the times suggested above your chances of having your snipe bid accepted by eBay before the auction closes diminish geometrically, perhaps exponentially. (I'll leave that to the AS mathematicians to calculate.) If you're not comfortable with allowing so much lead time just remember that proxy bidders are equally affected by the bidding traffic jams on Sunday. One big difference is that most of them don't know it.
Good luck!