How much time in Fiji? I want to learn to snorkel, dive, and do other stuff

[xpost from thorntree, seems a bit quiet there]

I'm in the process of planning an open-ended trip starting in Fiji and then moving onward to New Zealand, whereupon I'll probably get some more ideas on what to do next. I'm not on any particular schedule, but I do want to take advantage of Air Pacific's low rates for connecting to NZ through Fiji, since that beats buying separate tickets by a wide margin.

Objectives for Fiji, in priority order:

1. Learn to snorkel and dive, preferably at multiple sites. Would be nice to make it as far as an Open Water certification, and best if I can stay long enough to receive the C-Card after processing all the paperwork before moving on. I'm starting from nothing except knowing basic swimming -- I haven't tested how far I can do distance nowadays, but I've definitely been able to do 200m+ in a swimming pool (admittedly, a decade ago, when I was in my teens).

2. Hiking and trekking with sightseeing (e.g. Bouma National Park)

3. Visiting cultural sites (e.g. Fiji Museum) and traditional villages

4. Relaxing in the Yasawas or Mamanucas

5. Rafting the Navua

I'd like to be able to do at least #1 and #2 with enough "slack" in the schedule to have a very high confidence in actually succeeding at that.

I'm planning to head there at the beginning of January, so I'm aware that inclement weather can be problematic and may delay some of my planned activities. Tips on what can still be done when it's raining would be appreciated.

How much time should I budget for this leg of my journey?