http://en.wikipedia.org/wiki/List_of_English_Football_League_managersOnly 28 managers have been in their jobs for over 2 years...
Already, there are 31 managers who have been in their positions for less time than Gary Johnson has.
It really makes a mockery of giving a football manager a 3 year (or even 2 year) contract!
Not overly Cobblers related but its incredible reading when you take a look at the list. Below are the ones (not many!) who have been in their jobs for over 3 years. The old saying that continuity increases the likeness of success isn't necessarily the case when you run through the list...
Alex Ferguson 31 December 1941 Manchester United Premier League 6 November 1986 [1] 25 years, 4 days
Arsène Wenger 22 October 1949 Arsenal Premier League 1 October 1996 [2] [n 1] 15 years, 40 days
John Coleman 12 October 1962 Accrington Stanley League Two 10 August 1999 [3] 12 years, 92 days
David Moyes 25 April 1963 Everton Premier League 15 March 2002 [4] 9 years, 240 days
John Still 24 April 1950 Dagenham & Redbridge League Two 17 April 2004 [5] 7 years, 207 days
Tony Pulis 16 January 1958 Stoke City Premier League 14 June 2006 [6] 5 years, 149 days
Paul Tisdale 14 January 1973 Exeter City League One 26 June 2006 [7] 5 years, 137 days
Mick McCarthy 7 February 1959 Wolverhampton Wanderers Premier League 21 July 2006 [8] 5 years, 112 days
Terry Brown 5 August 1952 AFC Wimbledon League Two 15 May 2007 [9] 4 years, 179 days
Steve Evans 30 October 1962 Crawley Town League Two 29 May 2007 [10] 4 years, 165 days
Kenny Jackett 5 January 1962 Millwall Championship 6 November 2007 [11] 4 years, 4 days
Graham Westley 4 March 1968 Stevenage League One 2 May 2008 [12] 3 years, 192 days
Harry Redknapp 2 March 1947 Tottenham Hotspur Premier League 26 October 2008 [13] [n 2] 3 years, 15 days
Greg Abbott 14 December 1963 Carlisle United League One 3 November 2008 [14][15] 3 years, 7 days
By my reckoning, from the list of 14 above, only 5 or 6 could you classed as 'boxing above their weight and exceeding realistic expectations given budget etc'.
So that throws that argument out of the window to keep the current incumbent! Or indeed any manager at any given time.