I used to upgrade every 2-3 years but now I only have space for 49/50" its stopped me upgrading.I'm now waiting for 49/50" OLED to appear before I upgrade again.
I've had my last TV since 2010, a low end 42" LG Plasma (I forget the model number). It's been reasonably solid but it's definitely showing its age now.
I take delivery of a Sony ZD9 next month and I'll no doubt be keeping that at least 5 years otherwise the wife might divorce me!
I suspect it comes down to how good your current TV is. When I finally left the comfortable world of CRTs I bought an early 40" 1080i LCD, and it looked like utter garbage. Whilst it was larger and higher resolution than my old CRT, the picture was so awful I couldn't wait to be rid of it. Two or three years later I bought a 50" Kuro and that served me well for 10 years.
I've just bought a 65" B7 OLED and I hope it will see me through another 10 years.
In short, buy good stuff so you don't need, or even want, to replace it.
Had 3 TV's in 12 years and it may be a couple of years before I change again ... so averaging about 5 years between changes.
Factors in play have been:
12 years ago my first LCD panel - a 26in model from Philips that cost about £860 was very happy with it but only HD Ready so mostly fed with SD content
About 8 years ago to a 'BIG' LCD - a 42 in Toshiba (with a 10-bit panel) which was about as large as I could get at the time and costing £1080 - but had a lovely picure. Weakness the Freeview tuner on TV was just SD so bought a Freeview HD Recorder and upgraded to FreeSat HD tuner to feed it better images.
Then my current TV which a Samsung 50in JU6400 4K TV (2015 model and Cost £860) which having a more compact body as it does, only takes up about the same space as the older 42in. did.
This has a wonderful picture (to my eyes)which 4K content from Netflix etc. and my Blu-ray movies also look great. (I've posted some images here in the past)
If it has a weakness it is that it is only an 8-bit panel which on the odd occasion betrays the TV ie the openning view of the Earth in Space (Planet Earth II) or the odd vivid blue sky that would show 'posterization' ? For sound I have a Samsung Blue-tooth soundbar & sub combination.
With TV's getting more compact I believe I can just about fit a 55in TV whenever I upgrade again, maybe in a couple of years time - and that TV is likely to be a 55in OLED (12-bit and all the HDR, HLG etc. etc.) Hoping too that by then the price 'might' just under my limit of £2000
I last upgraded in I think 2013 and got a 47" Philips 6007, coming from a 42" 9664. Its a reasonable set, cost me £1000.Next set will be a 55" OLED however, money is tight so possibly in another two years it will be possible!
Only upgrade when there a significant technical advance or it breaks down, I have a B6 and very happy with it, had my previous Pioneer 5090 for 9 years
Got LG 60PX990 first ever THX 3D. still using it. Set me back at £3k at a time. But pic quality is still strong.
I bought a 42 inchpioneer 7 series many yrs ago then sold it for a 50 inch 5090 kuro which set me back over 2k had to get one as they were going to stop making them that was 8 yrs ago.. apart from the red tint which is now fixed it has been bulletproof... and still gives amazing picture..!!most reliable thing I've ever bought own a krp600 in my den too only thing that made me want to change in the past was azt65 but now an oled..!! but not enough money other priorities and people complain about niggles and other problems so am staying put till the pioneers die...
I used to average every 5/6 years but got a 50" Panasonic plasma in August 2007 which I finally decided to replace in May this year with a 65" LG E6 Oled. Hopefully that'll last me as long as the plasma did.
Up until fairly recently, I would only replace a telly when it went bang, but in 2009 I managed to persuade my wife that we should use some of our wedding vouchers on a Sammy UE40B7020 and a blu-ray player. That has kept going faithfully until last year, when we converted our garage into a playroom, and she suggested getting a telly to put in there. I suggested we'd be better off getting a 4k telly for the lounge, and moving the old one into the playroom. So now we've got a KS7000 (which came with the obligatory blu-ray player deal, this time a UHD one) and we decided not to put the old telly in the playroom, so it's up in the spare room, gathering dust. So, on current experience, once every 7 years...