At what sort of mileage should glow plugs be changed.

[dtmini]

My MK5 2.0TDI has been playing up by not starting some times. The car has done 50000 ks mostly long trips (I live in the bush) Thanks in advance,Dennis.

[stickshift3000]

I may be wrong but I believe they only need to be changed if a scan throws up plug error codes.

What sort of condition is your battery in? Diesels are particularly hard on batteries which can show up in symptoms similar to yours.