Change Dock Expose Activation Time
I'm not aware of a setting for just clicking on the icon with the mouse other than a default for turning it off...

defaults write com.apple.dock show-expose-menus -boolean

You can change the behaviour of when you drag a file onto a dock icon though...

Go to Finder Preferences and at the bottom of the general tab there is "Spring-Loaded folders and windows"

You can dis/enable it and change the delay time.