The PS3 & 360 were flukes as home consoles go. The Wii was more typical, peaking in its second full year. Here's the peaks by year & region for every console I have annual sales data on:

SNES: 1992, U.S. (1st year)
Genesis: 1993, U.S. (4th year)
Saturn: 1996, U.S. (1st year)
N64: 1997, U.S. (1st year)
PS1: 1998, U.S. (3rd year)
DC: 1999, U.S. (launch year)
PS2: 2002, U.S. (2nd year); 2002, Japan (2nd year)
GC: 2003, U.S. (2nd year); 2002, Japan (1st year)
Xbox: 2004, U.S. (3rd year); 2002, Japan (launch year)
Wii: 2008, U.S. (2nd year); 2007, Japan (1st year); 2008, Europe (2nd year)
360: 2011, U.S. (6th year); 2009, Japan (4th year); 2010, Europe (5th year)
PS3: 2011, U.S. (5th year); 2009, Japan (3rd year); 2011, Europe (4th year)

So, the norm for home consoles is to peak by the third full year. Only the Genesis, PS3, & 360 took longer, and those both have plausible explanations. The Genesis had to deal with Nintendo's de facto monopoly and didn't start to sell well until 1991 when Nintendo relaxed their licensing policies towards third parties. Meanwhile, a good argument can be made that the Wii was stifling growth of the 360 & PS3 as gamers chose the cheaper and more innovative system at first, but after it passed its peak gamers began switching back to systems that offered more traditional gaming experiences.

In the U.S., Nintendo systems have peaked either in year one (SNES & N64) or year two (GC, Wii, and possibly Wii U). The PS1 peaked in year three (it didn't start to really take off until after FFVII came out) and the PS2 peaked in year two.  The Saturn and Dreamcast both peaked very early, but they were never significant sales forces and were languishing behind their competition. The Xbox peaked in year three, and that's mainly because of a price cut and Halo 2. In Japan, hardware tends to peak earlier than it does in America. Meanwhile, Europe may more closely resemble America, with seventh-gen peaks almost perfectly mirroring those in the U.S.

As for handhelds, the DS peaked in year five in the U.S. but peaked in year two in Japan and year three in Europe. The GBA peaked in year two in the U.S. and launch year in Japan. The 3DS meanwhile peaked in all three major regions by 2013.

So, if I had to assign a norm for a system peak, it'd be year two or, less likely, year three. So, either this year or next year will be the peak for the PS4 & XBO, depending on the effects of price cuts and any potential system-sellers. With the Wii U being no Wii, I doubt they'll have delayed peaks like we saw with the PS3 & 360.

I see people repeating this over and over, I've never seen any evidence that supports it though.

Not trying to be mean or anything, but your writing style is somewhat difficult to parse (is English your first language?). I think I get most of what you're trying say, so here goes:

Sales from the 90s & 00s is still relevant today, because history reveals consistent patterns about why systems sell the way they do (not specific numbers, but relative sales as well as sales curves). Saying that only the previous generation is worth of consideration when discussing curren-gen sales is a claim that lacks any merit. And I don't appreciate the insinuation that I use historical data merely because it supposedly suits my personal opinions. On the contrary, I go where the data leads me, and historical precedent demonstrates that delayed peaks are rare and when they happen it's during unusual conditions.

The U.S., Japan, and Europe collectively represent 85% of global console sales. The U.S. alone represents 40% of global console sales and Europe about another 35%, so those two alone will influence the global peak for home consoles considerably. Furthermore, the economic situation has little bearing on console sales. PS3 & 360 sales actually peaked during the recession and overall game sales remained strong, demonstrating that the console market is largely recession-proof.

The 360 & PS3 were by no means lacking in strong titles early in the console cycle. By the time of the 360's third birthday in the U.S., the following games were released for the system: COD2, Oblivion, Dead Rising, Saint's Row, GRAW, Rainbow Six: Vegas, Gears of War, COD3, Crackdown, BioShock, Mass Effect, Halo 3, Assassin's Creed, COD4, Fable II, Fallout 3, Left 4 Dead, Gears of War 2, COD: World at War, and GTAIV. I could provide a longer list if you want. In any case, the 360 & PS3 were not hurting for good software early in their lives. So, your software argument doesn't hold up.

Finally, regarding your final point, by that reasoning sales should always continue to increase as the system gets cheaper, but that doesn't happen. The PS2's peak was when it was at $200. Further cuts to $180, $150, and $130 did not make it sell better than it did at $200. There are plenty of other examples, but in any case it's quite clear that the PS4 will likely find its peak before it gets to $200.

So, after looking at historical data on how systems sell, the most plausible causal mechanism for the delayed peaks of the PS3 & 360 was the presence of a certain inexpensive non-standard console that was setting the sales charts on fire during the first several years of the last generation. You have still yet to provide any actual data supporting the notion that peaks occuring 4+ years after launch are the new normal.