Stimulated nuclear polarization (SNP) in acyl-alkyl biradicals of different size formed as a result of alpha-cleavage of cyclic ketones in the triplet state has been investigated in low magnetic field around 10 mT. Time resolution was achieved by pulsing the light and the rf source and monitoring the SNP signal as function of the rf pulse delay. The SNP amplitude reaches its maximum for a delay around 40 ns and decays exponentially for longer delays. The decay time decreases with increasing molecular chain length. The different influence of tripler quencher cis-piperylene and radical scavenger CBrCl3 shows that the decay is mainly determined by the lifetime of the photoexcited ketone triplet state. The SNP maximum position can be attributed to the competition between spin-dependent and spin-independent channels of intersystem crossing.