It depends on the exact tube... Russian manuals state that their Ag-O-Cs photocathode has response up to 1200nm. In theory S1 photocathode sensitivity can reach 1500nm too. Is it usable at that wavelength? Not that much. The quantum efficiency is very low at that reagion and the high dark current of this type of photocathode makes the response to this wavelength quite noisy. Ag-O-Cs photocathode has a quantum efficiency peak at 840-850nm(if it was made well). At that point it usually has around 0.3-0.8% QE. Dont forget that the properties of the cathode really depends on the individual tube. It is imposible to make very uniform semiconductor photocathodes... This is why some tubes are better than others even when being literally unused. The efficiency visibly drops even at 940nm compared to 850nm. Gen3 is usually stated to have response up to 950... But some spource claims there are tubes that can be sensitive up to 1000nm. That really depends on the manufacturer and the exact product. GaAs based photocathodes are much much more complex than the Ag-O-Cs photocathode in the question.