There is no real 'last date' - which is probably why you're getting different answers.
First, go to the ETS website. The GRE testing website will give you a range for how long it may take to send your scores, normally they tell you a couple of weeks if I remember correctly (which is different from and additional time on top of the time it takes to get your full score report from the day you take the test). Furthermore, date variability exists because some universities are strict about receiving your scores by the deadline... whereas others will say that their deadline is Dec. 1 but that they will accept GRE scores until like Jan. 1. Also, remember that while most schools receive the scores electronically (the 'quickest' way) there are other schools out there that still receive scores via cd once a month or via paper from ETS (more old school methods). *You* have no control over that and it will be hard to find out which schools use which methods. Thus, the smartest (and easiest) thing to do is, once your scores are available, to request all of the scores at once online (and pay online) weeks ahead of time.
I remember when I looked into this to decide when was the last date I could comfortably take it, it was around mid Oct or so (in other words, accounting for the time it'd take to get the full score report *and* the time to send it). I think you can do the start of Nov. as well, but I was not willing to risk that. Even sending out my scores the recommended amount of time before the Dec. 1 and Dec. 15th deadlines (some program websites will recommend requesting the scores to be sent at least a month ahead of time), one or two of my schools still didn't receive my scores until a couple of weeks later than my other programs (i.e., longer than the range the GRE said it usually take). Those schools had me fax over a copy of my GRE score report for their records until they received them. As a side note, this is a prime reason for checking in a couple of weeks after the deadline to make sure everything has been received.
