When to replace glow plugs
 

2007 C30. 98000 miles.

I know it's how long is a piece of string but wondering when they need to be replaced.

Also if one goes can it be a safety risk please ?

With PSA engine you will get check engine. They are at the back of the engine and high risk of breaking during removal.

You test them with a multimeter and replace the dead ones or replace all at the same time on the basis that they will die soon too.