My last rebating attempt around xmas for mostly buy.com stuff (remember the great google deal?) was not a very successful venture. Out of about 15 rebates I had 6 of them messed around for various reasons and one I forgot to sign. 2 of those were supposedly post marked on the wrong dates. When I called about those, and they told me the postmark dates on the phone, I was happy to point out to them they were postmarked within their time frame. But one was supposeldy post marked like 3 weeks after I mailed it, LOL, which is impossible since I took those same rebates to the post office myself, all at the same time. Another was denied because it didnt fall with in the program dates (it did, and I had to point it out on the phone to get it). And another for $100 from Office Depot said in process for 6 months before they finally sent it to me after numerous calls (dont know what that problem was). Another was rejected because the UPC supposedly didnt match the one they wanted sent in (it did, but they decided they wanted a different UPC on the package after FRYs sneakily added their OWN UPC to the box elsewhere). All these problems had to be fixed with phone support, except the one I forgot to sign, I was able to resubmit that one. I finally got all those rebates, and I cant say I am eager to try that many again close together in the near future.
But to answer the question, usually I postmark them about in the middle of their due period. One thing to watch out for, though, is when they list an expiration date for the offer AND list something that says must be postmarked x days after purchase. So you may have 7-14 days to postmark it after purchase but the offer is good for a month or more. VERY SNEAKY.