Most people walk the Tongariro Northern Circuit in a clockwise direction, starting and ending at Whakapapa. I found three days to be perfectly adequate, though a lot of people split the first day at the Mangatepopo Hut some 9km from Whakapapa, making it a four-day hike of 9km, 10km, 15km and 7km. It all depends on how fit you are; the first section is pretty flat but it does suffer from erosion, and it's probably the least interesting part of the whole trek.
If you don't have time for the whole circuit then the walk from Whakapapa to the road past Ketetahi Hut makes a fantastic (though longish) day walk. It's known as the Tongariro Crossing and takes in the best of the Circuit, though if you have the time the whole Circuit is highly recommended. In beautiful weather, Ruapehu is a delight to walk under and makes the last day very interesting. Anyway, here's the route I took round the Circuit:
|1||Whakapapa to Ketetahi Hut||19|
|2||Ketetahi Hut to Waihohonu Hut||15|
|3||Waihohonu Hut to Whakapapa||7|
As The Tongariro Northern Circuit is one of New Zealand's Great Walks, you need to buy a hut pass in advance and the huts can be pretty full in peak times. Get in there early; this is one walk you don't want to miss.