Riddle me this, Riddle me that!

[Mister 44]

Okay don't get your nickers in knots please.... Here ya go:

There are three bags.The first bag has two blue rocks. The second bag has two red rocks. The third bag has a blue and a red rock. All bags are labeled but all labels are wrong.You are allowed to open one bag, pick one rock at random, see its color and put it back into the bag, without seeing the color of the other rock.

How many such operations are necessary to correctly label the bags ?

This rings familiar. I have seen something similar somewhere before. Don't think it was in this thread, though.
Anyway, I won't answer unless nobody else gets it.