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On land it may take hundreds of years. I am sure that it deteriorates much quicker in sea water and becomes less likely to detonate but dependent on certain factors that may take a very long time. How oxygenated the water is must play a part, as does the depth of the munition under water, as does the thickness of the steel casing. Depth has a bearing on pressure meaning that water will more rapidly find a way into the munition. Thickness of the casing: I have seen that steel has a grain, hence the use of shells fitted with base plates. Seawater etches out the softer parts of the steel and degrades the strength of the whole. I have found milk bottle-sized lumps of rust and one that was broken revealed a 20 mm driving band. I realised then that I was looking at a fired practice shell. The steel was all gone, leaving only rust and the copper driving band. I have found an expended 25 Pr smoke shell - recognisable by a blunt forward end (aerodynamic tip broken off or rotted away) and a slight concave look to the wider rear despite the thickness of the rust. When I removed the rust what was left of the fuze fell out of the shell. The steel fuze well had rusted away completely, probably because it was the thinnest part of the steel, it was threaded and had had a threaded hole for a grub screw to secure the fuze. A 25 Pr plugged shell (in WW2 there was a training dispensation that allowed HE plugged shell to be fired seawards) had been deeply indented by the application of a demolition charge but had not detonated. That said there was a hole below the shoulder so it may perhaps have deflagrated when detonation was attempted. The hole was not as a result of the use of a shaped demolition charge.
