Check the actual auction carefully and the seller's information (profile or terms), he may state somewhere he has 7-10 days or more to contact you and request payment. If he states this anywhere you will be obligated to pay.
I have some very recent ebay selling experience. Our account clearly states (if you look it up) that if an item is purchased Fri-Sun you will not hear from us until Monday, otherwise our response is (I believe, but forget we are quick lol) 3-4 business days. So if you bought from us at 12am Friday it is feasible you will not hear from us until Monday at 11:59pm and you would still have to pay, thats just about 4 days.
If you have not heard put in a request for the sellers information, ebay will let the seller know that you have requested the info...it will give you something to fall back on. Also if you do not hear leave the seller a negative feedback saying that you never heard after winning the auction...that is often motivation for a seller to get their act in gear because negative feedback can deter bidders quickly.