reftex-auto-view-crossref

reftex-auto-view-crossref is a variable defined in `reftex-vars.el'.
Its value is

t

Documentation:
Non-nil means, initially turn automatic viewing of crossref info on.
Automatic viewing of crossref info normally uses the echo area.
Whenever point is idle for more than `reftex-idle-time' seconds on the
argument of a \ref or \cite macro, and no other message is being
displayed, the echo area will display information about that cross
reference. You can also set the variable to the symbol `window'. In
this case a small temporary window is used for the display.
This feature can be turned on and off from the menu
(Ref->Options).

You can customize this variable.