Tooltip fade out delay in OS X 10.11.1 and Xcode 7.1.1
You can set it up via Apple Terminal. For example:

defaults write com.apple.dt.Xcode NSInitialToolTipDelay -int 5000


The value of delay is measured in milliseconds. So 5000 ms == 5 sec.

To find changed NextStep ToolTipDelay values in definite domains use:

defaults find NSInitialToolTipDelay