Via Wikipedia's page List of Prime Ministers of New Zealand by term (a page I admit to starting) John Key will pass Jenny Shipley tomorrow to become New Zealand's 23rd longest serving Prime Minister, out of a field of 38. It will also mean Key passes all the Prime Ministers who were interesting 'footnotes' of history (except Sir George Grey, who is more famous for being Governor twice, and Norman Kirk who died in office).
From here it's another 97 days before Key passes 19th century Premier John Ballance, who also died in office. From then on it's another 4 years (and at least two elections) before Key will pass another National Party Prime Minister, Jim Bolger who is currently ranked 10th longest-serving. Shortly after Bolger comes Ward, the master of the political comeback, and two further big names in 20th century politics - Muldoon (7th) and Holland (8th).
However, Key will need to be in office for another 12 years and 39 days to beat the record holder, Richard John Seddon at 13 years and 44 days.