Wut?
If you do it properly, it's in 2WD straight away (4WD light flashing).
If you didn't do it properly, after a few metres (before I get down my driveway to the edge of the road) the light comes on solid and the car is in a 50/50 split.. really not good to drive around like this!
If you've been driving along in 4WD and pull over to put it in 2WD mode, it might take 30 seconds or so to relieve all the pressure.