10 days: North Island only
A realistic loop without a single long transfer day. Start and end in Auckland: 2-3 nights in Auckland itself, 2-3 nights in Rotorua for the geothermal valleys and Māori cultural centres, 2-3 nights in Wellington for the capital's museums and food scene, and the remaining days in the Bay of Islands for a slower, warmer finish. Driving between each stop is comfortably under four hours, leaving real time at each destination rather than losing days to the road.
10 days: South Island only
The classic South Island loop: fly into Christchurch, spend 2 nights there, then drive south to Queenstown (roughly a full day's drive, or fly direct) for 3-4 nights as a base for Milford Sound and the region's adventure sports. From Queenstown, head north along the west coast to Franz Josef and Fox Glaciers for 2 nights, then loop back to Christchurch. This route covers the island's headline landscapes without the rushed feeling of trying to also fit in the North.
14 days: one island, properly
The same routes as above, with the extra four days spent slowing down rather than adding new stops — an extra night in Queenstown for a side trip to Milford Sound without the same-day round-trip pressure, or an extra day in the Bay of Islands. Fourteen days on one island, done properly, tends to be more satisfying than fourteen days split thinly across both.
21 days: both islands
A genuine full-country trip. Roughly 10 days on the North Island (Auckland, Rotorua, Wellington, Bay of Islands), then the Cook Strait ferry from Wellington to Picton, followed by roughly 10 days on the South Island (a shortened Christchurch-Queenstown-Milford Sound-glaciers loop). This is enough time to see both islands' genuine highlights without every day being a transfer day — but it does mean an abridged version of each island's own 10-14 day itinerary above, not the full version of both.
Shorter "both islands" trips aren't recommended
Attempting both islands in 10-14 days usually means cutting destinations down to rushed half-day stops and losing multiple days to transfers and the ferry crossing. If time is under two weeks, this guide's honest recommendation (see How to Decide) is to pick one island and do it properly.