How long brakes last is up to you
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I'm not certain I understand your question. If the aircraft is down and you need a part, then it's down and you need a part. This is true of any airplane. Whther a part is broken and needs replacement, or it's worn and needs replacement, what's the difference? Needs replaced is needs replaced.
Parts availability can sometimes be a difficulty. Getting brake assemblies runs in cycles. These aren't field rebuildable; these get sent back, and are expensive. They're expensive enough that operators tend not to stock them as rotables. It's a specialized carbon fibre assembly. My opinion on the brakes is that judicious use of reverse thrust and feather during taxi, proper power management, and allowing the airpalne to rollout on landing instead of heavy braking will contribute to long brake life. On the other hand, I've seen crews wear them out fairly quickly.
Avantair's problems were largely self-made...a failure to stock parts when that many common aircraft were in use was a mistake; aircraft were constantly being canibalized to keep each other flying...their parts management was mostly musical parts among the fleet, and the problem was that when aircraft came in for work, they'd get ravaged to keep other aircraft flying, and then couldn't be released right away. Another common issue there was a failure to address maintenance issues right away, or to overlook them, leading to bigger problems and frequent grounding. A small department operation shouldn't face these issues.
In fairness to Avantair, they operate quite a few hours on an annual basis, and never end up twice in the same place at night except by coincidence. The aircraft arrive at a maintenance base only for scheduled inspections or when something goes wrong...the aircraft does not have nightly attention by the maintenance department. The company has changed hands and operations philosophies several times, which has left the maintenance department (and flight department) in a constant state of change...which also has contributed to the down time. I don't know that as a corporate or charter operator of the type, you'd see nearly the downtime or difficulties that they have faced...again, they're the largest single operator of the type in the world.
The flip side of the coin is that as the largest operator, one might suppose they would get preferential treatment for parts and service, but this isn't usually the case.
Their aircraft are flying often in the order of eighty hours or more a month each, sometimes double that.
My impressions of the aircraft were favorable. I believe any corporate department which has experience stocking and maintaining an inventory and maintenance program for their aircraft won't find the Piaggio any more difficult or different to maintain. There's no reason why it can't be made to have a high reliability rate. The downtime I saw with Avantair isn't necessary, and the program could clearly have been much improved. That isn't as easy done as said in an organization of any size, or one that's micromanaged as it was...the aircraft are capable of being more reliable and being operated more efficiently, in my opinion.
I did receive reports from time to time within the maintenance department that some components were difficult to obtain from Piaggio. I can't verify this personally, as I wasn't involved in ordering parts.
Piaggio tends to use small wiring, with multiple splices, and multiple wires into terminals that aren't normally seen on US aircraft. I do know of occasions when this became an operationla issue with locating faults in splicing, unions, grounds, etc. The size of the wiring makes it a little more susceptible to damage or failure, though this is also preventable through proper handling.
The airplane flies nicely; for turbojet performance it operates on a turboprop budget with DOC's being reasonably low (can't provide specific figures, but roughlly comparable to a BE-200, but faster, higher, and quieter). As I said before, customer preference as strongly in favor of the Piaggio when presented with alternate choices; the customers loved them.