Needs["DifferentialEquations`InterpolatingFunctionAnatomy`"];
Clear[sol]
sol = NDSolve[{y'[x] == -0.2 y[x], y[0] == 10}, y, {x, 0, 100},
Method -> {"EventLocator", "Event" -> y[x] - 1 == 0}];
end = Max[InterpolatingFunctionDomain[First[y /. sol]][[1]]]

sol = NDSolve[{x'[t] == a*x[t] - c*x[t]*y[t], x[0] == 10,
y'[t] == -b*y[t] + d*x[t]*y[t], y[0] == 10}, {x[t], y[t]}, {t, 0,
tend}, Method -> {"EventLocator", "Event" -> y[t] - 1 == 0}];

end = InterpolatingFunctionDomain[y /. First[sol]]

InterpolatingFunctionDomain[y]