It does depend pretty heavily on the way schools' adcoms work as well.
For example, UCSF is pretty much done for this year because they only do interviews til end of January and I believe just about all of the invites are out now (unless ppl who got in elsewhere cancel their interview). But for schools like USC and Columbia (the two that are sort of known for doing things somewhat later in the cycle) are still inviting ppl for interviews.
But as I've said before, if you haven't heard back from any schools even for interviews, you're probably in the third round. Usually second round is for people who interviewed in december/january and they usually get their acceptances sometime in mid~late january when schools find out how many seats are still open after the january 15th deadline passes for the dec 1 acceptances.
The third/fourth/fifth rounds I would say is probably from somewhere now til may with the last round being may ~ school starts. The process is as I've mentioned different for each school. Some schools like to send out batches of acceptances while some send out one acceptance at a time as people decline their offers.
Hope this helps.
