TMax=123;
Needs["DifferentialEquations`InterpolatingFunctionAnatomy`"];
ySol=y/.NDSolve[{
y''[t]==-Sin[y[t]],
y'[0]==0,
y[0]==0.99π
},
y,
{t, 0, TMax}][[1]];
Plot[ySol[t],{t, 0, TMax}]
tVals=Flatten[InterpolatingFunctionGrid[ySol]];
δtVals=Table[ tVals[[i+1]]-tVals[[i]],{i,1, Length[tVals]-1}];
ListLogPlot[δtVals,
AxesLabel->{"step n","Δt"},
PlotRange->All]