todo-done-separator-string

todo-done-separator-string is a variable defined in `todo-mode.el'.
Its value is

"="

Documentation:
String determining the value of variable `todo-done-separator'.
If the string consists of a single character,
`todo-done-separator' will be the string made by repeating this
character for the width of the window, and the length is
automatically recalculated when the window width changes. If the
string consists of more (or less) than one character, it will be
the value of `todo-done-separator'.

You can customize this variable.