desktop-auto-save-timeout is a variable defined in `desktop.el'.
Its value is

Number of seconds idle time before auto-save of the desktop.
The idle timer activates auto-saving only when window configuration changes.
This applies to an existing desktop file when `desktop-save-mode' is enabled.
Zero or nil means disable auto-saving due to idleness.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.