The walk to the Rob Roy Glacier viewpoint is 3 hours return not including time out to take photos or whatever (and there are plenty of opportunities). Although I dislike crowded walks and on this day it was fairly busy I didn't mind in the slightest because the views are magnificent. The walk is mostly uphill but it is very gradual so you don't notice so much. When we got to the glacier view point we hung out eating our lunch and fending off the kea - they are VERY persistent and cheeky and will take off with anything they can carry if you are not watching them carefully.
There is a huge waterfall opposite the viewpoint (which appeared in a locally famous base jumping video called 'The One') it is truly an amazing site. You can make your way across the river quite easily and take a shower under its spray - refreshing!
After eating lunch, my friend and I felt that we had lots of energy to burn so we decided to carry on until we got straight under the glaciers. That was another hour and a half without a track. We had a look around, took some photos and headed back. Well worth the effort. Including lunch and breaks the whole walk took 7.5 hours. That's from Raspberry Flats car park to viewpoint: 1.5 hours, viewpoint to glaciers: 1.5 hours, and then stoppage time and return to Raspberry flats carpark.
For the amount of effort that this walk takes the rewards are great and very inspirational.