Hello,
Sounds like a great trip! To be honest, it does depend on where you stay in Fiji as to what would be too long a trip. Fiji is a beautiful destination however a month may be too long if you're sticking with the standard type of tourist experience. I can advise that with staying in local communities, you don't have the sameness that you get in backpacker hostels and dorms, you really find out the uniqueness of the place you are at in Fiji, the people, the culture, the traditions, and of course, the traditional foods! So yes, a month could be great for you if you are interested in travelling around Fiji and exploring the different local communities. There is also heaps more to do in Fiji at a fraction of the cost when you stay in local villages, you can make great friends and also have access to some of the more remote places in Fiji that are off-limits to normal tourists. At all times the locals are your hosts, and the money you pay when you get to the village actually goes towards the communities themselves. If you are interested at all in this option, go to www.fijibure.com to learn more about it - it is likely you'll be turned away if you just turn up without arranging your stay in advance through that website...you are initiated as a member of each village you stay in, you meet the chief of the village, it is pretty unique! It is really the only option available in Fiji where you can truly say you've "experienced" Fiji if you know what i mean. So check it out if you're interested. Here are maps of fiji with the different villages which have opened up to homestays -
Viti Levu is here at this link: http://www.fijibure.com/map.htm
Yasawa Islands is at this link: http://www.fijibure.com/yasawa.htm
Good luck, I hope you have a wonderful time whatever you end up doing!!