Never, unless you play indoor golf.
A true fitting can only be accomplished if you're doing it on the same conditions you will be playing on.

2 reasons to decide on an indoor facility for a fitting. Mats are not the same on turf interaction. Shot monitors add a lot of assumptions into launch, peak height, and descent angle based on 12 feet of ball travel.
Especially with wedges, it is difficult to be able to take the results outside and achieve the same response.
I’m a confirmed believer in an indoor SAM lab for a putter fitting. Takes out the variables not directly related to my stroke. After that, give me grass.

Agree with Don, outdoors is ideal but I will also add, if indoors is all that is available besides just buying a set off the rack, do the indoor fitting. You can at least get a good idea for basics like length, lie angle, shaft firmness.

I have my first official indoor fitting in 2 weeks with a Titleist rep. I played all winter on a Simulator, and I can say that the Trackman will under-estimate the distances that you will see on a real course. I assume all the other data is accurate.