Experimental measurment of thiosulphuric acid decomposition kinetics
Measuring conductivity over time might do the trick (7 ions (8 charge equivalents) in, 4 ions/charge equivalents out).

Regarding spectrometry: you could deliberately make use of the light scattering caused by the precipitating sulphur. Should be somewhere in the 600+ nm range. It's a standard method in biological sciences to follow the growth of bacteria.