10.5 Years.

To calculate the average time between assassination attempts for all notable U.S. civil, political, and other influential leaders (including presidents, politicians, and prominent civil rights figures), we need to expand the list beyond political figures to include:
Political Leaders (Presidents & Politicians)
Andrew Jackson - 1835 (attempt)
Abraham Lincoln - 1865 (successful)
James Garfield - 1881 (successful)
William McKinley - 1901 (successful)
Theodore Roosevelt - 1912 (attempt)
Franklin D. Roosevelt - 1933 (attempt)
Harry Truman - 1950 (attempt)
John F. Kennedy - 1963 (successful)
Robert F. Kennedy - 1968 (successful)
Civil Rights and Other Notable Leaders
Martin Luther King Jr. - 1968 (successful)
George Wallace - 1972 (attempt)
Gerald Ford - 1975 (two attempts)
Ronald Reagan - 1981 (attempt)
Barack Obama - 2011 (attempt)
Donald Trump - 2024 (2 x attempt)
Additional Notable Figures
Malcolm X - 1965 (successful)
Harvey Milk - 1978 (successful)
This gives a total of 18 notable assassination attempts/assassinations between 1835 and 2024. Now, I'll recalculate the average time between these events.
Including all civil rights leaders, political leaders, and other notable figures, the updated average time between assassination attempts is approximately 10.5 years. ​​