Monarchies in Commonwealth Realms

Until the mid 60's, every British colony that was granted independence from the United Kingdom became a "Commonwealth Realm" monarchy by default. Queen Elizabeth II was recognized as the newly independent nation's Head of State and sovereign monarch with a distinct title (ie; "Queen of Pakistan").

Although the British government retained no authority over nations in which Queen Elizabeth was monarch, many locals in these new nations found the continuing presence of the British monarchy in their system of government to be an unnecessary holdover of colonial rule.

From 1956 to 1992 Queen Elizabeth was stripped of numerous titles as more than a dozen nations changed their constitutions to become republics. Rather than force more nations to go through this hassle in the future, from the late 60's onward all British colonies that were granted independence became republics by default, rather than constitutional monarchies under the British Queen.

There are a total of 17 former realms that are now republics:

 
Became indepenent
Time as Commonwealth Realm
Governor
Generals
Monarchy abolished
First President
Ireland 1922 15 years, 3 d.
3
December 29, 1937 Newly appointed
India 1947 2 years, 5 months, 11 d.
2
January 26, 1950 Newly appointed
Pakistan 1947 8 years, 7 months, 8 d.
4
March 23, 1956 Incumbent Governor General
Ghana 1957 3 yeas, 3 months, 26 d.
2
July 1, 1960 Newly appointed
South Africa 1910 51 years
12
May 31, 1961 Incumbent Governor General
Tanzania 1961 1 year
1
December 9, 1962 Incumbent Prime Minister
Nigeria 1960 3 years
2
October 1, 1963 Incumbent Governor General
Uganda 1962 1 year
1
October 9, 1963 Newly appointed
Kenya 1963 1 year
1
December 12, 1964 Incumbent Prime Minister
Malawi 1964 2 years
1
July 6,1966 Incumbent Prime Minister
Guyana 1966 3 years, 8 months, 24 d.
3
February 22, 1970 Incumbent Governor General
The Gambia 1965 5 years, 2 months, 6 d.
2
April 24, 1970 Incumbent Prime Minister
Sierra Leone 1961 9 years, 11 months, 22 d.
5
April 19, 1971 Incumbent Governor General
Sri Lanka 1948 24 Years, 3 months, 18 d.
4
May 22, 1972 Incumbent Governor General
Malta 1964 10 years, 2 months, 23 d.
2
December 27, 1976 Incumbent Governor General
Trinidad &Tobago 1962 13 years, 11 months, 1 d.
7
August 1, 1976 Incumbent Governor General
Fiji 1970 16 years, 11 months, 27 d.
3
October 7,1987 Newly appointed
Mauritius 1968 24 years
7
March 12, 1992 Incumbent Governor General

Referendums:

Before they broke with the monarchy, several nations held national referendums on whether or not their citizens supported the adoption of a republican constitution.

 
Date of referendum
Yes vote
Ireland July 1, 1937
56.5%
The Gambia November, 1965
62.3%
Ghana April 17 - 30, 1960
88.47%
South Africa October 5, 1960
52.3%
Australia November 6, 1999
45.13%

Australian voters rejected adopting a republic in 1999. As such they are technically the only Commonwealth Realm to have democratically chosen their status as a monarchy.

The Survivors:

Today 15 countries remain Commonwealth Realm monarchies under the British Queen. Prior to the reign of Queen Elizabeth II they were known as "Dominions" of the Crown.

 
Time as Commonwealth Realm
Became dominion / realm
Population (appx)
Canada 138+ years July 1, 1867
33,000,000
Australia 104+ years January 1,1901
20,000,000
New Zealand 98+ years September 26, 1907
4,000,000
Jamaica 43+ years August 6, 1962
2,700,000
Barbados 39+ years November 29, 1966
280,000
The Bahamas 32+ years July 10, 1973
300,000
Grenada 33+ years February 7, 1974
89,000
Papua New Guinea 30+ years September 16, 1975
5,700,000
Solomon Islands 27+ years July 7, 1978
550,000
Tuvalu 27+ years October 1, 1978
11,000
St. Luca 26+ years February 22, 1979
17,000
St. Vincent & The Grenadines 26+ years October 27, 1979
12,000
Antigua & Barbuda 24+ years November 1, 1981
69,000
Belize 24+ years September 21, 1981
290,000
St. Kitts & Nevis 22+ years September 13, 1983
39,000

Summary

Canada, Australia, New Zealand, and South Africa were the first Commonwealth Realms, and remained realms for the longest period of time. Jamaica will have to stay under the Queen for at least another seven years to beat South Africa, and I don't think that's very likely.

Here are Queen Elizabeth's four longest-held titles, and thus the four nations she should devote most of her Queenly energy towards:

Queen of Canada 53+ years
Queen of Australia
Queen of New Zealand
Queen of Jamaica 43+ years
Queen of Barbados 39+ years

The present-day republic in which she reigned the longest as Queen was Mauritius (24 years) followed by Sri Lanka (20 years).