It's an interesting question Terry - and one that, being so hypothetical, doesn't lend itself to an easy or definitive answer. Yet one that can hopefully be addressed without displaying nationalistic myopia, flashes of which can be seen elsewhere.
The question perhaps has two parts:
1. How long could the allies have maintained an effective war footing without American support, even if just in materiel?
And:
2. Regardless of 1., how long could Germany maintain its defence - if not offence - in the face of the strictures it was subject to, at home, let alone on the battlefield?
How long is a piece of string?
We could argue the logistical and motivational strengths of the opposing forces until the cows come home; but it seems likely that the attritional depletions suffered by Germany as a consequence of the blockade would have proved too much to bear within perhaps another two years (a guess, of course).
The allies may not have had much in the way of offensive capability in such a protracted period; but their lock on Germany would have at least seen it sue for peace in time.
Whether this would have been acceptable to the allies on terms, or whether they would have pressed for an unconditional capitulation based on a sense of their own 'longevity' in a prolonged conflict is highly moot; but it was only going to end one way.
This is also being discussed here, with a few hints of temper too:
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Best wishes,
GT.