I was thinking about what I consider as silliness for people who buy say a PS4 then PS4Lite or PS4Pro. Same with those who bought XboxOne then bought the S and plan on getting Scorpio. But then I started thinking about own buying habits.

Nov 2006 - Wii
Feb 2010 - PS3 (3yr 3mo)
Nov 2012 - WiiU (2yr 9mo)
Oct 2015 - XboxOne (2yr 11mo)
Nov 2016 - NES Classic (1yr 1mo)
Mar 2017 - NS (4mo)

That is not including any PC upgrades / rebuilds I did (2010 and 2015). So I guess, it is not silly really. I'm just thinking about it differently as I'm not buying the same platform more than once. However, if NS has a new version come out 2yrs out that is more powerful, it is very likely I'd consider buying that system. Especially if 3rd party support is there.

With that in mind, how often do you buy new gaming hardware? (can just do last couple gens)

Gaming PC (2012)
PS3 Super Slim (2012)
WiiU (2014)
PS4, 3DS, Vita (2016)

There was a period when I bought more, but as I got older and had more responsibilities and less time I've slowed down and just tend to have larger libraries on platforms I enjoy.

Recent Systems
Nintendo 3DS - August 2012
PlayStaion 4 - November 2013
PlayStation Vita - December 2014

I debated for months on the Wii U in 2015, with Zelda: Breath of the Wild being the game I waited on for years. Now that I know it will be on Nintendo Switch I will likely get that system which will also have ports of several of Wii U titles that appeal to me. I have played the system at a friends house, enjoyed Smash and Mario Kart.

I also have debated on the PS4 Pro, but was saddened that it wouldn't play 4K movies because I wanted it in part to show off 4K television and sound system I've been planning on buying. If they add that in and enhancements to games is significant, I'll consider it strongly.

Future Purchases?
PS4 Pro / PSVR - Maybe 2017/2018 if I see the value in them
Nintendo Switch - Likely 2018 or if a pretty Zelda themed bundle is released in 2017.

Whenever my console master decrees it time.

PS2 (3 years)
Gamecube (Launch)
Wii (6 months after launch)
Xbox360 (2 years)
PS4 (1 year and 6 months I believe)
DS (I honestly don't know when, much later after DSlite)
PSP (Sometime before PSP3000 or at PSP3000)

3DS (Launch)
WiiU (Launch)
PS4 (Launch)
Vita (Five months ago)

Since the 3DS, I've bought all my consoles at launch. So that's the 3DS, Vita, Wii U and PS4, and as long as I have the money, I'll probably get the Switch at launch too.

Before the 3DS, I didn't really have a job, so it took longer to save up, or I had to wait to get it as a present.

GameCube: I think 2 years after it came out
Wii: 2 months after launch (because of console shortages)
PS3: 1.5-2 years after launch
360: 3 years after launch, I believe

Depends. The last I bought was a PS4 in May, before that I bought a gaming rig in July 2015. Before that was another gaming rig in like 2009 and a PS3 in 2009. I guess one device every 3-4 years on average, but it was 6-7 years between the last gen purchases.
Next for me is probably a PS4 Pro when the price comes down or depending on the actual performance, would be nice to get some more use of my 4K TV, I'll buy it straight away if a version or update appears with 4K blu-ray capability.

There's really not anything else planned, Scorpio is off the plate since I don't care about most Xbox exclusives and most big titles will come to PC anyway, the Switch is still undecided for me, the form factor and concept is more or less the worst Nintendo could have presented for my part but if the games are awesome and the performance is okay I might get one when the prices come down.

Not very.

I buy as much new hardware as I can. My father bought us the Atari VCS. The first console I personally bought was a used NES. Since then, every console I buy is new. It might not be new as in "just released" but as in I'm the first once to remove it from the box.

As often as possible.

2012 New Gaming Rig GTX 680
2013 PS4
2014 Wii U
2015 GTX 980ti
2016 New Gaming Rig GTX 1080
2016 PS4 Pro
2017 Nintendo Switch

Something like that.