It will be interesting to see how long people keep supporting the older platforms, and I suspect the type of game will come into play - essentially we're already down to "hand-me-down" titles (people give their old platform to a young sibling/friend, so traditionally things aimed at a younger audience (or historically female) tend to do well for a while longer). However, since trade-in is now so common now there's probably a lot less of that these days.
Supporting the older platforms generally holds you back - PS3 is pretty much dead now and 360 is going that way... Certainly at retail (I guess you might continue doing digital if your game already supports the older generation platforms)...
I'd think that by this time next year it will be all but done except for budget titles.