You can use a standard method of getting running totals using Sumif, combined with Match:
=ArrayFormula(match(sumif(column(S1:Z1),"<="&column(S1:Z1),S1:Z1),sumif(column(S2:Z2),"<"&column(S2:Z2),S2:Z2))-1)
I put rows 3 and 4 in just as a check of my calculations and to show the results of the two Sumifs evaluations - they aren't necessary.
You may wish to specify what should happen if the demands add up to exactly 3000, for example. The above formula would actually go to the next month, so may need some refinement if that is not what you want.
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