Pred is a short term drug and typically it is given twice a day,
sometimes three times a day.  For some ferrets Prednisone doesn't work
but Prednisolone does.  That happens when a ferret's liver isn't up to
the processing step which the former needs but the latter doesn't.  For
some, also, Diazoxide is a useful drug.  It is NOT unusual for dosing to
have to be adjusted (and adjustments are needed throughout the illness).
The usual technique is to try a low dose first and up it if is doesn't
hold.  Call your vet.
 
What other tests were done?  CBC WITH Chemistry Panel?  Imaging?
 
I advise
http://fhl.sonic-weasel.org ,
and also some of the vets' pages listed in the critical refs in
http://www.ferretcongress.org
especially
http://www.afip.org/ferrets/index.html
 
The stress itself of illness and of surgery has been known to start
ulcers.  There recently was an edition of "Ferrets" magazine with an
excellent article by Dr. Bruce Williams on this topic.  Ask your vet
is trying Carafate would make sense for your ferret.  Pretty much any
pharmacy can fill a prescription for the liquid version which ferrets
often accept very readily.  It is unusual to have inappetence continuing
this long after surgery of the growth was a milder one.  What were the
pathology results from the biopsies?  Has an adrenal problem been
considered?
 
Here are some past mammary tumor (apocrine, hyperplasia secondary to
adrenal tumor, etc.) vet comments:
http://fhl.sonic-weasel.org/browse.php?msg=SG4105
http://fhl.sonic-weasel.org/browse.php?msg=SG468
http://fhl.sonic-weasel.org/browse.php?msg=SG448
http://fhl.sonic-weasel.org/browse.php?msg=YG9111
http://fhl.sonic-weasel.org/browse.php?msg=YG4682
http://fhl.sonic-weasel.org/browse.php?msg=SG7005
http://fhl.sonic-weasel.org/browse.php?msg=SG6389
[Posted in FML issue 4488]