Your "My ebay" search results only go back 90 days (if you don't delete the listing), so I would think this would be the cut off. I actually received a feedback reminder the other day, but couldn't find the listing in "My eBay". When I tracked back when I listed it, I found it was ~3 months old. I can't tell you for sure, just what I've experienced.
Yes this has definitely changed to 60 days. There were some items that were still on my list and I wanted to jog the buyers memory to leave me fb so I tried to leave it for them but the system would not let me. Even though the item was still on my list the 60 days had expired so I couldnt leave it,
{As buyer} is there a period for me to leave feedback before I'm not able to? 
