I dont think its about how long the job will take, From what you said in your OP it's incase the part is wrong when it turns up.
that way they can order the right one without messing you about.
But if you want to be travleing backwards & forwards IF the part does come in wrong then just book it in for 1 day. your choise
Any chance you work for a dealer

I hope this is just because the fan assembly etc is particularly complicated on the GTI and therefore not possible to order correctly for certain. A it's my first VW I hope I don't need to take it into the garage for 2 days everytime something needs doing.
Anyway, this is a bit immaterial now, here's the latest twist.....
The car went into a different dealer today for the same fault. At 3.00pm I get a call saying that they've investigated & there's no fault with the fans - both are working. They suggested however it was probably needing a re-gass which would be causing the coolant to overheat & setting the large fan off. I commented that the small fan had definitely not being working, confirmed by the 1st VW dealer. Apparently this wasn't the case (even though I know for certain it was).
My cynical guess now is that the mech today found a faulty connection/connector which now means the fans are ok but the dealer tried to squeeze £65 out of me for the privilege. My first experiences with VW dealers don't look too promising!!!