...how long are these effective? My ferret got one in March. 

It seems to depend on the ferret somewhat.  They are supposed to be good for a year, but Jaffa got one in March as well and it looks like he's finally growing some fur back.  
http://www.virbacferretsusa.com/about-suprelorinf-implant

THE DOOKS OF HAZARD

About 1 year, but it depends on the ferret.  Our Rocky got one in February, so is due in February again, but we may give him one in January (or December) if he looks like he might need it sooner (difficulty urinating, etc).  I think the big boys are the ones that might need it sooner rather than later.

Interestingly enough, Rocky was on Lupron until the Des implants came out.  Rocky was on a double dose of Lupron due to his size.  Now, using the Des, I think he might need his sooner than most as well.  

Ah. Nito's hair was totally grown back within a month or so of getting the implant. I had forgotten that she had a little white spot on her chest :) Question - how does treatment change as she gets older and the disease becomes more advanced? Do I just keep getting the implants and treat symptoms as they come? I know surgery is an option...my vet was pretty opposed to it in March because the condition wasn't very bad. I haven't noticed any progression (or really any symptoms) since about a month after the implant. Before that she had a good amount of hair loss, excessive thirst, and very dry skin and fur...

As an aside, she seems to have to urinate frequently (but isn't drinking really excessively like before). I would say at least when she comes out to play she goes once every hour or so. Is this related to adrenal or just a thing?

I think the implant is the best course.  Kahla only had 2 in about 5 years but had a nice coat when she passed at 8 years old.  We've given Jaffa a couple melatonin implants to see it they will boost the fur regrowth.  Surgery is very risky on the right adrenal gland.

THE DOOKS OF HAZARD

That's good to know. So does the vet do some kind of titer to determine when the implant needs to be replaced, or is it the return of symptoms?