There's probably not that much activity so early on Sunday morning. I'd say 10 seconds would be reasonable for your snipe. The 15-second timing for Sunday night should be OK too, but I'd recommend that you do a simple test perhaps 10 minutes prior to the auction close by doing an eBay search (for anything) and checking out one or two items resulting from that search to see how quickly, or slowly, eBay reacts. If it's as slow as molasses in January, add another 3-5 seconds. And, of course, allow at least 20 seconds for any auction ending on an even hour, half-hour, or quarter-hour (XX:00:00, XX:30:00, XX:15:00, or XX:45:00) regardless of what day of the week it is. Good sniping!
