Since 1958 and over 5,000 episodes!
(so much of classic UK TV is either "has been broadcasting since the dawn of television in a largely unchanged format" or the other extreme of "we only ever made six episodes back in the 1970s and they are etched across everyone's childhood")
Or alternatively sometimes scarred for life :)
And if taking a trip back through 1970's UK TV is something anyone's interested in then this is a fun book:
https://www.lulu.com/shop/stephen-brotherstone-dave-lawrence...
(This doesn't always follow, of course. The UK version of The Office is only 12 regular episodes plus a couple of specials, yet it's impact is still very much felt - at least in the UK - so I don't necessarily see there being a simple correlation between quantity of material and cultural significance. As much as the US series is also excellent I don't think it carries quite the same iconic status or cultural weight here even though there are a lot more episodes - obviously it might do in the US.)
W and I cannot stress this enough AT?!!!!!!
We had multiple re-runs of it in the 90s in various post-Soviet countries.
It's the artistic equivalent of Step 1: take these household materials, Step 2: ???, Step 3: Profit!