This was in the faqs (seems a little conservative from what the forum recommends):
1. How do I add a snipe?
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Lead Time
Lead Time is the amount of time in seconds from the end of the auction that you would like to snipe. A 5 second lead time will bid when there are just 5 seconds remaining. Auction Sniper recommends a 5 second lead time. Add 2-4 seconds more during peak times (5-10 p.m. PST and weekends).
10. I sniped at the right time! What happened to my snipe?
If you look at a snipe confirmation, you might see as an example:
Lead Time 5
Snipe Time 02/10/2002 17:13:50 -5 sec
Actual Snipe 02/10/2002 17:13:50 0 sec
Auction Sniper will send your bid to eBay exactly when we agree to, on the right date at exactly the right time with a 5 second lead time. After we send your bid, it can encounter internet traffic and not reach eBay in time. In addition, eBay may not be able to process your bid in time during peak hours: you might want using slightly higher lead times between 6 and 10pm PST.
64. What is the ideal lead time to use for a snipe?
Lead time is the number of seconds from the end of an auction that you would like AuctionSniper to bid for you. We recommend lead times between 5 and 10 seconds, although we've seen times as low as 2 and 3 seconds work just fine. Currently, Auction Sniper successfully places 99.9% of all snipes with 5 second lead times.
We don't recommend anything less than 5 seconds Although we send the snipe on time, during peak times an eBay server may not process it in time. This is not the fault of eBay or us; internet traffic is just heavier at certain times. And during peak periods, you might consider adding 2-4 seconds more since their servers seem to be taking quite a hit during that time (5-10 p.m. PST), and on weekends.
I still like your pop-up window idea to warn users for auctions ending in heavy usage periods.