Dave,
I meant wait until the wine is dry or quits bubbling for the most part.
Once all of the sugar that can be converted is consumed by the yeast
you will literally see the activity cease. The wine will also begin to
clear since there is no turbulence.

I disagree on the 'throw the hyrdometer away' comment, if you are
making wine from kits you may not need it but it's the best $10 you
will spend if making wine from anything else.

You will eventually develop a sense of taste that gets you close but
the device is pretty foolproof.

Joe