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?