It is a difficult question to answer, without making assumptions about the manner in which the Borg might go bust.
Firstly, I think Watchtower to "go bust", the US branch would have to go bust, in its own right. Whatever happens in the rest of the world is irrelevant by comparison.
Secondly, it would make quite a difference whether the "bust" is due to a default with a financier, or just a failure to keep paying salaries and various operating expenses. I don't think I have ever seen any evidence that Watchtower has borrowed money, so I would assume the latter.
If the above 2 assumptions are correct, it would be difficult for the US branch to go bust in a spectacular manner. Instead, would would see them become late payers on everything, shutting down any significant expensive operations (eg printing), reducing funding for overseas missionaries, laying off bethelites, and selling off non-core assets (ie underutilized Kingdom Halls, etc). That is pretty much what they are doing now (except I don't know whether or not they are late payers).
If the above is the actual situation (which is really just a cash flow issue), then it could go on for decades. It would not create any sudden drop in adherents, but it would be corrosive to the numbers over time.
If (hypothetically) the US branch does have significant financial debts, and it were to default, it may not become public knowledge, unless the default is spectacular, in some way. I don't see that happening, but if it did, perhaps it could cause some adherents to peer behind the curtain. Unfortunately, the majority would probably just follow whatever explanation the GB gave.