The injection time tau of the electron transfer from an excited electronic state of a dye molecule to a semiconducting surface is studied, An analytical expression for the injection time as a function of excitation energy is derived. The semiconductor is described as a one-dimensional chain in the tight-binding approximation, and the dye molecule is included via the coupling to the semiconductor. For a TiO2 semiconductor the injection time is estimated to be approximately 5 fs. The results support and characterize recent experimental studies.