Almost all visitors to Fiji arrive at Nadi International Airport, with many transferring directly to resorts along the Coral Coast or to one of the Mamanuca or Yasawa islands. Overland travelers intent on exploring Fiji on their own should start moving the morning after they arrive. Save your sightseeing around Nadi until the end of your trip.
Two weeks is the absolute minimum required to see a good cross-section of Fiji, and with even a month at your disposal, you’ll barely scratch the surface of all there is to see and do. You’ll need a week with stops to travel right around Viti Levu, then another week for Vanua Levu and Taveuni, and a third week could easily be spent in the Yasawa Islands. Of course, you could do it all faster, but your transportation costs will be the same and you’d be rushed. After a month in Fiji, you’ll be in a position to begin planning your next visit.
However, if your only aim is a beach holiday or some focused scuba diving or surfing, a week may be enough. You should consider booking a package tour to control your costs and avoid wasting time. With only a few days in Fiji, you won’t see much more than Nadi and environs. To make the most of your time, you could use one day for a daytrip to one of the Mamanuca islands and another to go to Sigatoka and back.
You should also make an effort to explore beyond the resorts; to discover the “real Fiji” in the marketplaces, city streets, and back roads of this vibrant country. Traditions run deep in Fiji. The cultural programs presented for visitors at the beach resorts are played out in earnest back in the villages. You might witness Fijians walking briskly over white-hot stones in a steaming pit, or Indo-Fijians treading on glowing coals during a religious firewalking ceremony. Or your Fijian hosts might invite you to join them for kava or yaqona drinking. (Muddy, musty kava is a taste you may or may not acquire during your stay.).
In short, how you plan your trip will depend on who you are. If you’re primarily a beach person, spending most of your time at one resort may work just fine. If you like being active, check carefully which nautical sports are available at your resort. If you want to see more of Fiji than the beach, pick somewhere on one of the main islands rather than an isolated island resort. On the other hand, the outer islands tend to have far superior beaches.