Longest career as an understudy to the same role

Longest career as an understudy to the same role
Who
Nancy Seabrooke
Where
United Kingdom (London)
When
On 12 March 1994, Nancy Seabrooke, retired from the Company of The Mousetrap at the age of 79 after having understudied the part of Mrs Boyle for 15 years and 6,240 performances. She performed the part on 72 occasions.