The short answer: the minimum amount of samādhi needed for the lowest level of nobility, stream entry

At the very minimum, a finger snap's moment of first jhāna is needed for stream entry (guaranteed nirvana in 7 lifetimes or less).
While there is no sutta that explicitly says it like that,
it's a necessary conclusion from both deduction and inference after reading all the suttas.
Otherwise, how could an eightfold path, the only path that leads to nirvana, mention 4 jhānas under right samādhi, 

but neglect to mention there is a special kind of inferior samādhi (below first jhāna) that leads to nirvana?