Karl Marx did not use the phrase, socially necessary labour time (or its equivalent, labour time [that is] socially necessary) in the Grundrisse (1857-58 notebooks). He did, however, refer once to "the necessary labour of society":
As soon as labour in the direct form has ceased to be the great well-spring of wealth, labour time ceases and must cease to be its measure, and hence exchange value [must cease to be the measure] of use value. The surplus labour of the mass has ceased to be the condition for the development of general wealth, just as the non-labour of the few, for the development of the general powers of the human head. With that, production based on exchange value breaks down, and the direct, material production process is stripped of the form of penury and antithesis. The free development of individualities, and hence not the reduction of necessary labour time so as to posit surplus labour, but rather the general reduction of the necessary labour of society to a minimum, which then corresponds to the artistic, scientific etc. development of the individuals in the time set free, and with the means created, for all of them. Grundrisse, page 705.
As will become clear in my future discussion of the originally unpublished "Chapter Six" of Capital, Marx was referring to something that had already happened, "modern industry" or "the real subsumption of labour under capital," and not to some hypothetical event in the future. The surplus labour of the mass had already ceased to be the condition for the development of general wealth. But what about labour time and exchange value ceasing to be the measure of use value?
Moishe Postone saw this passage as pivotal:
Given the distinction between value and material wealth, so long as the production of material wealth depends largely on the expenditure of direct labor time, both "necessary" and "surplus" labor time can be considered socially necessary.
This, however, ceases to be the case as the production of material wealth comes to be based on socially general knowledge and productive capacities rather than on direct human labor. In such a situation, the production of material wealth may bear so little relation to the expenditure of direct labor time that the total amount of socially necessary labor, in both its determinations (for individual reproduction and for society generally), could be greatly reduced. The result, as Marx put it, would be a situation characterized not by the "reduction of necessary labour time so as to posit surplus labour" but rather by "the reduction of the necessary labour of society in general to a minimum." Time, Labor and Social Domination, page 374.
I will take the opportunity to plug my publication, "The Ambivalence of Disposable Time" in each of these episodes. I am linking to the published journal article. If anyone needs free access to the author's preprint, let me know in comments and I will leave a link there.