The default time it takes for a tooltip to show on hover (In the MS
world) is 500 ms (Cooper also describes this as the acceptable delay).
That value is drawn from the mouse double-click time in the control
panel. You can tweak it however you like for your app, though. In the
app I work on we needed to slow tooltip display significantly because
we were bombarding the user.
The standard for placement of the tooltip WRT a text field is that it
be centered underneath the field itself. The control has built in
support to make this happen (This is opposed to tooltip for a button
which floats around to make sure it appears on-screen). This is drawn
from the Windows User Experience Guide.
I have at least one example showing that tooltips show when hovered
over the name of the text field and not the field itself, but do not
have a definitive answer on that point. Hope this helps,