We recently sent in 2 - within a week of each other. Each of them took about a week total - from when I registered it, recd the envelope, sent in next day and within 4-5 days we had the replacement ipod. Much faster now than when we sent one in last November.
However, reminder - the warranty on the new one is 90 days. One of our replacements would only hold a charge for about 6 hours. Not being familiar with this model, I just figured it was a smaller/weaker player. Well, it decided to "die" on day 91 it wouldn't recognize in itunes or limited charge. I called them, they emailed me things to try - reset, etc. Tried suggestions for 2 days, then called them again. Since it was past the 90 days, they nicely explained to me, sorry, it was out of warranty. I heated up a bit and didn't give up. They actually went back to the FedEx records to find the date that I signed for the receipt of replacement and then said they'd honor it, but impressed upon me that it was an exception only with supervisor approval.
So, pay attention to your new unit to make sure it's functioning properly. Remember, it's a refurb so may have had a different issue originally.