eBay only allows feedback to be left up to 90 days after an auction ends.  They use a sliding window, so it's never exactly 90 days from the time the auction ends, but it's within a few hours of the end date of the auction, which makes it hard to feedback snipe.  Here are links to more info on how the feedback system works on eBay:

Is sniping allowed on eBay?  There are no rules prohibiting either what time you leave feedback or place a last minute bid on eBay.  eBay actually defines sniping in their glossary:

Sniping
Placing a bid in the closing minutes or seconds of an auction-style listing. Any bid, placed before the listing ends, is allowed on eBay. To protect yourself from being outbid at the last moment, enter the maximum amount you're willing to pay for an item up front, and eBay will bid automatically for you, making sure you're the high bidder until your maximum is reached. This system is sometimes called proxy bidding.

Most eBayers are familiar with sniping, at sites like AuctionSniper.com.  There are currently no sites available for automating the process of leaving last minute feedback.  In order to feedback snipe, FeedbackSniper.com recommends:

• Find the exact end date of your auction.  Look at the eBay auction page and it will display a field called: "Ended"
• Use a calendar program to set an alert for 89.5 days after the exact end date of your auction.  Check to make sure this alert will happen at a time you'll be awake and able to receive the alert.
• Decide at the point you receive your alert when the best time to feedback snipe is.  Waiting until closer to the exact 90 day mark will be ideal.

Actual eBay user comments:

"I have the following problem: I have a problem leaving negative feedback at ebay (when it has to be). ebay allow 90 days after the transaction for feedback. I always try to leave negative feedback 10 minutes before the 90 days are up so the other ebay member has no chance to leave me negative feedback.  Sneaky, but effective. It is quite difficult and cumbersome to calculate the 90 days from the end of the transaction."

"will someone out there please invent FEEDBACK SNIPER SOFTWARE that wallows one to leave feedback (good or bad) at the last second? that way, you can leave legit bad feedback w/ no fear of retaliatory bad feedback left for you- thus making ebay a better community as a whole. I do not leave deserved bad feedback for fear of retaliatory bad feedback left one me!!!"