I just made a cross-check at http://maps.aa.co.nz/ for distances and travel times. Giving the routing Christchurch - Hokitika - Wanaka- Queenstown which reflects the routing along the West Coast - gives me a distance of about 740 km and a driving time of over 9 hours. And that does not yet reflect that you will see curvy roads and hard to pass traffic (e.g. motorhomes) which will increase your driving time to well above 10 hours. That gives you really little room for anything to see or do - and there are lot of interesting things.

For comparision: The shortest distance is 500 km and something above 6 hours.