Question: How long does the prime minister which has Sarla Grewal as the principal secretary serves?

Context paragraphs:
Principal Secretary (Canada): In Canada, the Principal Secretary is a senior aide, often the most senior political aide, to a head of government.  Formerly, the position of Principal Secretary was the most senior in the Canadian Prime Minister's Office, though since 1987, it is second to the Chief of Staff.  The Leader of the Official Opposition and most Canadian provincial Premiers also have a principal secretary.

Pulok Chatterji: Pulok Chatterji (born 1952) is an Indian civil servant who served as Principal Secretary to the Prime Minister of India, Manmohan Singh, from 2011 to 2014.  and formerly served as Executive Director to the World Bank and Joint Secretary at the Prime Minister's Office from 2004 to 2009.

Swedish governmental line of succession: The Swedish constitution of 1974 allows the Prime Minister of Sweden to appoint one of the Ministers in the cabinet as Deputy Prime Minister ("biträdande statsminister", also unofficially known as "vice statsminister", "Vice Prime Minister"), in case the Prime Minister for some reason is prevented from performing his or her duties.  However, if a Deputy Prime Minister has not been appointed, the Minister in the cabinet who has served the longest time (and if there are several with equal experience the one who is oldest) takes over as head of government.  Note that the person acting as Prime Minister does not do so on a permanent basis: if a Prime Minister dies, resigns or loses a vote of confidence in the Riksdag, the Speaker of the Riksdag will then confer with the parties of the Riksdag and propose a new Prime Minister, who must be tolerated by a majority of the Riksdag.  If the Prime Minister has resigned or lost a vote of confidence, he or she will remain the head of a government "ad interim" until the new Prime Minister assumes his or her office.  The only case where the governmental line of succession becomes relevant is when the Prime Minister dies (upon which the person next in the line of succession serves as the head of a government "ad interim") or when the Prime Minister is on leave or for any other reason incapable of serving, but still remains in office.  This might be compared to the Presidential line of succession in the United States, where the person next in line assumes the Presidency throughout the remainder of the term if the President dies, resigns or is impeached.

Sarla Grewal: Serla Grewal (October 4, 1927 – January 29, 2002) was the second female Indian Administrative Service officer in India, when she joined IAS in 1952.  She was the Governor of Madhya Pradesh (1989–1990).  She was the principal secretary for Rajiv Gandhi.  She died because of Pulmonary Tuberculosis and Chronic Renal Failure.  In addition to the above-mentioned posts, she also held Shimla's first deputy commissioner, Secretary to Prime Minister at WHO and UNICEF.

Prime Minister's Office (India): The Prime Minister's Office (PMO) consists of the immediate staff of the Prime Minister of India, as well as multiple levels of support staff reporting to the Prime Minister.  The PMO is headed by the Principal Secretary, currently Nripendra Misra.  The PMO was originally called the Prime Minister's Secretariat until 1977, when it was renamed during the Morarji Desai ministry.

C. R. Krishnaswamy Rao: C.R. Krishnaswamy Rao (2 February 1927 – 12 February 2013) was a distinguished administrator and civil servant.  He also served as the Principal Secretary to Prime Minister Charan Singh and then becoming the Cabinet Secretary (1981-1985) when Indira Gandhi became the Prime Minister of India.  He was awarded the second highest civilian honor of India, Padma Vibhushan in 2006, besides the "Great Maratha Award" in 2009.  His grandfather was R. Ramachandra Rao, Collector of Nellore, who helped Srinivasa Ramanujan during latter's formative years.

S. K. Misra: S. K. Misra (born 1932) is a retired Indian civil servant, social worker, writer and a former Principal Secretary to Chandra Sekhar, the 8th Prime Minister of India.  He served as the principal secretary to three chief ministers of Haryana and is a former secretary at the ministries of Tourism, Civil Aviation and Agriculture.  He is one of the founders of Motilal Nehru School of Sports, Rai and the founder chairman of "Indian Trust for Rural Heritage and Development".  The Government of India awarded him the third highest civilian honour of the Padma Bhushan, in 2009, for his contributions to Indian civil service.

Secretary of State for Northern Ireland: Her Majesty's Principal Secretary of State for Northern Ireland, informally known as the Northern Ireland Secretary (Irish: "Rúnaí Stáit Thuaisceart Éireann" ), is the principal secretary of state in Her Majesty's Government with responsibilities for Northern Ireland.  The Secretary of State is a Minister of the Crown who is accountable to the Parliament of the United Kingdom and is the chief minister in the Northern Ireland Office.  As with other ministers, the position is appointed by the British monarch on the advice of the Prime Minister.

Chief Cabinet Secretary: The Chief Cabinet Secretary of Japan (内閣官房長官 , naikaku-kanbō-chōkan ) is a Minister of State who is responsible for directing the Cabinet Secretariat of Japan.  The main function of the Chief Cabinet Secretary is to coordinate the policies of ministries and agencies in the executive branch.  The Chief Cabinet Secretary serves as the government's press secretary, conducts policy research, prepares materials to be discussed at cabinet meetings, and, in time of national crisis, coordinates ministries and agencies of the executive branch.  The Chief Cabinet Secretary is customarily nominated as the first in line to serve as temporary Acting Prime Minister in case the Prime Minister is unable to serve due to death or other grave reasons until a new Prime Minister is appointed.  The Chief Cabinet Secretary's office is located on the fifth floor of the Prime Minister's official residence in Tokyo.

Rajiv Gandhi: Rajiv Ratna Gandhi ( ; 20 August 1944 – 21 May 1991) was the 6th Prime Minister of India, serving from 1984 to 1989.  He took office after the 1984 assassination of his mother, Prime Minister Indira Gandhi, to become the youngest Indian Prime Minister at the age of 40.

Supporting paragraphs: Sarla Grewal Rajiv Gandhi
Answer: 1984 to 1989