What was or is the longest running Lego set? What is the longest running Lego set, retired or still in production? 

I remember seeing set 7239 Fire Truck in the Lego catalog for years. That fire truck out lived three fire stations (7240, 7945, 7208)!

I'm having trouble finding an end date for sets, is there any way to know how long a set was released?  

For example Brickset lists the fire truck's availability at LEGO.com as Oct 2007 through Dec 2011, but I know the set ran longer then that (I got it in 2005).

So what is/was the longest running Lego set?
I'm going to stretch your definition of "set" and say "32x32 green baseplate". You could get set 745-1 in 1978...



...then 840-1 in 1980, then 813-1 in 1986, then 626-1 in 1996:



And since 2015 there's 10700-1, altough it seems that the shade of green is different than the previous iterations:



Since those sets have exactly the same piece count (one) and each piece is functionally the same, I will argue that the green 32x32 baseplate set has been in production since 1978 until today (2019). That's over 40 years.

If you consider these to be different sets because they have different set numbers, then the record goes to 1996's 626-1, with 19 years.