Knowledge is power.
Book time quote is between 2 & 2,5 hours.
That quotation is based upon a minimal difficulty to remove them without busting them off !
It includes a possibility of a re-seal exercise around the intake system aft of the turbo etc.
Now the object is not to break them!
But it can happen so experience comes into play.
Question the shop service writer on things like how many have they successfully fixed /replaced?
Have they got any RECENT customer phone numbers to give you an idea of their skill.
If they have good workshop management program they should be able to whip you off a list of past customers willing to give you a referral.
Now in the case they do break ask them if they have the special tools to extract them.
If they don't--- run!
The best tools for the job are by Klann or Muller Vertzeug. If they don't have these tools at hand --run!
These tools are expensive and easily break.
The tool kit starts at $350 a set.
Now if we get vans/jeeps in with broken glow plugs the charge is $300 per hole.
Extra if someone has broken say a drill bit off or easy-out in the hole with ham fisted techniques . That then requires us to use a mini EDM machine or Dremel tool & diamond bit to cut it all out.
Its cheaper than lifting heads and bench work.
There is no hard and exact rule on successful extraction, time/patience/experience/preparation is a good aid to successful removal and not going at them like a bull at gate.
Hope that text is helpful to you.
Dennis