While we have never had a vet operate due to numbers alone for insulinoma,
once there are symptoms present they DO operate rapidly and we've been VERY
happy with the results.  The only times that the ops have not been rapid
ones over the many years have been when Fritter had it and people didn't
know what to do way back then plus she also had lympho, and for two ferrets
which had to delay for health reasons --one to be built up some first and
another due to other serious illnesses also present (though in her case the
surgery finally had to be risked and worked fine).  The points others are
making about time involved, borderline numbers, etc.  are excellent ones
and smack dab on the mark.  Remember that early symptoms can be as mild as
"distracted" periods with inactivity, or more sleepiness than earlier.
Don't forget to read up on the problem in:
 
1. http://www.ferretcentral.org
2. http://www.afip.org/ferrets/index.html
3. http://www.miamiferret.org/fhc
[Posted in FML issue 2977]