Elizabeth II is only the second British monarch to reign for 60 years (after Queen Victoria). George III just missed (reigned 59 years and 4 months).
Victoria reigned for 63 years and 7 months, which is longer than that of any other British monarch and the longest of any female monarch in history.
We shall see if Elizabeth can surpass her record.
The longest reigning monarch ever recorded was King Sobhuza of Swaziland from 10 December 1899 to 21 August 1982.
I doubt if Elizabeth can reign that long...