From MSDN:
  There are multiple delay values that
  you can set for a Windows Forms
  ToolTip  component. The unit of
  measure for all these properties is
  milliseconds. The InitialDelay
  property determines how long the user
  must point at the associated control
  for the ToolTip  string  to appear.
  The ReshowDelay property sets the
  number of milliseconds it takes for
  subsequent ToolTip strings  to appear
  as the mouse moves from one
  ToolTip-associated control to another.
  The AutoPopDelay property determines
  the length of time the ToolTip  string
  is shown. You can set these values
  individually, or by setting the value
  of the AutomaticDelay property; the
  other delay properties are set based
  on the value assigned to the
  AutomaticDelay property. For example,
  when AutomaticDelay is set to a value
  N, InitialDelay is set to N,
  ReshowDelay is set to the value of
  AutomaticDelay divided by five (or
  N/5), and AutoPopDelay is set to a
  value that is five times the value of
  the AutomaticDelay property (or 5N).
For more information on this subject, look here:
ToolTip Class
Make tooltips remain visible for a very long time in Visual Basic .NET
Can I set an infinite AutoPopDelay for a tooltip in a .net winform?
Tooltips with infinite timeout?