Question: Since when has the rank of James Cassels been at its current level in the British Army? 

Context articles: James Cassels (British Army officer)
Furir
Robert Cassels
General (United Kingdom)
Lieutenant (British Army and Royal Marines)
Major-general (United Kingdom)
Brigadier (United Kingdom)
Field marshal (United Kingdom)
List of British Army full generals
Military Secretary (United Kingdom)

Field Marshal Sir Archibald James Halkett Cassels, {'1': ", '2': ", '3': ", '4': "} (28 February 1907 – 13 December 1996) was a senior British Army officer who served as Chief of the General Staff, the professional head of the British Army, from 1965 to 1968.
 As a young man he was a first-class cricket player, initially playing in India for the Europeans against the Hindus in the Lahore Tournament and going on to play for a Punjab Governor's XI against Northern India team and for a Viceroy's XI against the Roshanara Club.
 He later played for the British Army cricket team against the RAF at The Oval and then played for the Egyptian national side against HM Martineau's XI in Alexandria.
Furir (from French "fourrier", a person responsible for feed) was a Swedish military rank above Korpral and below Sergeant awarded after completing 11 months of level 8 conscript training.
 Level 8 means that the holder has received some basic leadership training at team leader level.
 Riflemen, MP men and deputy team-leaders (5 men) of cavalry and riflemen at the Nordic Battle Group typically have this rank [ref: K3, K3, K1, NBG].
 Other holders of the rank are the deputy team-leaders of the Home Guard.
 In Sweden, the rank corresponds to Lance Corporal in the USMC and the British Army.
General Sir Robert Archibald Cassels, {'1': ", '2': ", '3': ", '4': "} (15 March 1876 – 23 December 1959) was an Indian Army officer.
 He was the father of Field Marshal Sir James Cassels.
General (or full general to distinguish it from the lower general officer ranks) is the highest rank currently achievable by serving officers of the British Army.
 The rank can also be held by Royal Marines officers in tri-service posts, for example, General Sir Gordon Messenger the new Vice-Chief of the Defence Staff.
 It ranks above lieutenant-general and, in the Army, is subordinate to the rank of field marshal, which is now only awarded as an honorary rank.
 The rank of general has a NATO-code of OF-9, and is a four-star rank.
 It is equivalent to a full admiral in the Royal Navy or an air chief marshal in the Royal Air Force.
Lieutenant (Lt) ( ) is a junior officer rank in the British Army and Royal Marines.
 It ranks above second lieutenant and below captain and has a NATO ranking code of OF-1 and it is the senior subaltern rank.
 Unlike some armed forces which use first lieutenant, the British rank is simply lieutenant, with no ordinal attached.
 The rank is equivalent to that of a flying officer in the Royal Air Force (RAF).
 Although formerly considered senior to a Royal Navy (RN) sub-lieutenant, the British Army and Royal Navy ranks of lieutenant and sub-lieutenant are now considered to be of equivalent status.
 The Army rank of lieutenant has always been junior to the Navy's rank of lieutenant.
Major general (Maj Gen), is a 2-star rank in the British Army and Royal Marines.
 The rank was also briefly used by the Royal Air Force for a year and a half, to August 1919.
 In the British Army, a major general is the customary rank for the appointment of division commander.
 In the Royal Marines, the rank of major general is held by the Commandant General.
Brigadier (Brig) is a senior rank in the British Army and the Royal Marines.
 Brigadier is the superior rank to colonel, but subordinate to major-general.
 While the corresponding rank of brigadier general in many other nations is a general officer rank, the British Army considers it a field officer rank.
Field Marshal has been the highest rank in the British Army since 1736.
 A five-star rank with NATO code OF-10, it is equivalent to an Admiral of the Fleet in the Royal Navy or a Marshal of the Royal Air Force in the Royal Air Force (RAF).
 A Field Marshal's insignia consists of two crossed batons surrounded by yellow leaves below St Edward's Crown.
 Like Marshals of the RAF and Admirals of the Fleet, Field Marshals traditionally remain officers for life, though on half-pay when not in an appointment.
 The rank has been used sporadically throughout its history and was vacant during parts of the 18th and 19th centuries (when all former holders of the rank were deceased).
 After the Second World War, it became standard practice to appoint the Chief of the Imperial General Staff (later renamed Chief of the General Staff) to the rank on his last day in the post.
 Army officers occupying the post of Chief of the Defence Staff, the professional head of all the British Armed Forces, were usually promoted to the rank upon their appointment.
This is a list of full generals in the British Army since the Acts of Union 1707.
 The rank of general (or "full general" to distinguish it from the lower general officer ranks) is the highest rank currently achievable by serving officers in the British Army.
 It ranks above lieutenant-general and below field marshal which is now only awarded as an honorary rank.
 The annotation "Held rank in the East Indies."
 indicates that the officer served in India in the East India Company's army.
The Military Secretary is the British Army appointment of which the incumbent is responsible for policy direction on personnel management for members of the British Army.
 It is a senior British Army appointment, held by an officer holding the rank of major-general.
 The position of Deputy Military Secretary is held by an officer holding the rank of brigadier.
 The Military Secretary's counterpart in the Royal Navy is the Naval Secretary.
 The Royal Air Force equivalent is the Air Secretary.