Experimental ratios between the emission rates of light fragments (Li, Be, B and C) from ^112,(116)Sn and ¹²⁴Sn targets are compared to predictions from a model which combines the Boltzmann master equation to describe the dynamics and preequilibrium particle emission and the statistical multifragmentation model to describe the final breakup. Since secondary decay and the choice of freeze-out volume affect these ratios very little this comparison can be used to estimate the excitation energy at the instant of fragmentation in a self-consistent way. The preequilibrium nucleon emission governs the time dependence of the excitation energy available for fragment production which is nearly the same for all Sn isotopes. Therefore the yield ratios can be used to estimate the fragmentation time. Data from 1 GeV proton- and 200 MeV ⁴He induced reactions indicate short fragmentation times - 20-40 fm/c while 32 A MeV ¹⁴N induced reactions seems to exhibit much longer times - 300-400 fm/c.