What is the shortest time to snipe a bid?

Generally it's dependant on the speed (latency, mainly) of your net connection.

Depending on your DSL and how loaded your connection is, you can probably go down to around 7-10 seconds.

With JBidwatcher I've sniped at 1 second on a REALLY fast connection, but it was a corporate internet connection, lots of bandwidth, super-low-latency, and an item I wasn't too concerned about losing.

I don't recommend anything near 1 second for home use; clock skew, other programs slowing the system down, random emails coming in, or any other internet traffic can make it so that a really close snipe fails.

In fact, I generally advise against going closer than 5 seconds, and even that is VERY close, and any variation in your local network traffic or CPU availability can slow the snipe just enough to miss it.

If you want to be relatively safe, stick with 10 seconds on the very fastest DSL lines, and increase it from there, capping out around 20-30 seconds on a slow dial-up connection. DSL is lower-latency than cable, so cable should generally start around 15 seconds at the fastest. (N.b. I have been informed by some that in non-US countries the Cable/DSL latency is reversed, and Cable is sometimes lower latency, so...your mileage may vary.)

You'll notice that JBidwatcher loads some pages at about 2 minutes + the snipe time before the end. It's not actually sniping, it's preparing the bid page, so all it has to do is submit one page at the last moment, to complete the bid. This is tuned to go as fast as it can.

All that said, sometimes eBay flubs a page-load, and doesn't respond, hanging the connection for 10-20 seconds, before it times out. When that happens, there's nothing to do. It's why JBidwatcher can't guarantee it'll successfully snipe; the variation of things that can go wrong is far too large. It tries its best, though.