30m since I wouldnt consider the N64 a failure. It barely had any 3rd party support and had to rely mostly on Nintendo and Rare.

100 million

lower than 30million => fail.

In the absence of any frame of reference I would say 30 million is the threshold. But if you take past generations into account then success or failure would be more.complex. To me a rule of thumb would be half of the immediate past gen sales = not failure.

This is completely subjective. Failure for the manufacturer? PS3 was a much bigger failure than Wii U.
For the consumer? Depends on his taste alone.

From a manufacturer's point of view, instead of a fixed number, I believe there are 2 factors that need to be considered:

First, if the console maintains or gains market share (long term goal).

Second, if it makes a profit (short term goal).

I believe that the balance between these two factors is what determines whether a console is a failure or not. If a console gains market share and earns a profit, it is a success. I think if a console gains market share but has a net loss, or if it loses market share but turns a profit, it can still be a success, as long as the loss in market share or profit is not huge. But if a console loses market share and has a net loss, then it is a failure.

Using these metrics, I think the original XBox and the 360 were successes. The PS3 was a failure. Unless things drastically change, the WiiU will be a failure.

From a consumer point of view, a console is a success if it is fun to play.

The gamecube made money? or Nintendo made money with the GBA and DS while the GC was on sale?

Financials in the black. (most important by far)

EDIT: Also, strategic factors like increasing or holding on to market share or outlasting a competitor.

Numbers don't say much.

Both, but the talking point that the "GC made more money than the PS2" has always been false. That refers to Nintendo's sales of its handhelds during the same time.

Which is why its hard to determine a consoles success by a balance sheet. Even the Nintendo, the least complex of the game companies, has multiple products.