Laura,
 
I'd be concerned about waiting for a couple of months; if that spleen
ruptures, your ferret will bleed to death within minutes.  We had one MF
ferret who was diagnosed with an enlarged spleen at about the same age.
Our vet removed the spleen (in the nick of time, judging by its size) and
the ferret made a rapid recovery and was quite healthy for another 7 years.
 
You might want to get a second opinion about whether it's safe to wait for
further developments.
 
ellen & the Gang of [for the moment] Three
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[Posted in FML issue 2434]