I change mine every other year, which is about 10,000 miles,
One usually looks like new, but the other one is possibly a little cleaner than yours, but noticeably different to new.
Mike. G reg 2.5VM Vogue Portofino red 1991- 1999
V reg 2.5td P38 Rioja red 1999- 2006
53 reg td6 Vogue Oslo blue 2006- 2015
11 reg 4.4 TdV8 Vogue SE. Baltic blue 2015- date.
https://www.fullfatrr.com/forum/topic56162...tty+affair

Thanks - I have no idea how quickly the get dirty, but it seems like it wasn't changed recently, so I'll get those done as well - and assume the fuel & cabin filter also need doing.