A friend just brought to my attention, this blog post which I made in 2011, during National's first term:
And if National does manage to win a second and third term, then i do think that John key could do what no other prime minister in NZ's history has done – and get out while on top. I don't think he wants to set a record for longest serving PM in power as Clark did. I think he will feel that 7.5 to 8 years as PM and 9 – 10 years as National Leader is a pretty good run, and he'll let new leadership emerge for that elusive chase of the 4th term.
I knew I had talked generally about Key wanting to go out on top, but had forgotten I had been so specific as to timing.