In a 10 day period, I was able to make a complete loop around the south island taking in time to see much of what you are stating. That being said however, I think you might want to give yourself some "wiggle" room. I personally found that what I thought I wanted to do for X days ended up going X+1 or sometimes X-1. I'd give yourself some gray area to make adjustments as the situation on the ground dictates (and as your experiences go).