timing guides

[Ryoken]

some background.
as some of you might know I've had an oil leak on my 01 m62 for a very long time, several rocker gaskets later, and its still present ,
the other day I took the car to my works workshop ( work at royal mail, I went to the RM fleet workshop)(they service ALL RM vehicles) any whoo , he diagnosed upper timing case gaskets , not rocker gaskets.
the question I have is this , how long on average do timing guides last?
are there any owners who are on their original ones ,and at what mileage you on?
my engine is on 160K ,. serviced regularly ,3 year old timing tensioner,
doing the rockers are easy, I've done them enough times, upper timing cases don't seem hard, but I don't want to re do them all , if a lot of people say 160k is high for original guides, as , you change quides the mechanic , who does this job will be replacing all the gaskets ive just done

if you get my meaning
cheers
Steve