Electron transfer (ET) dynamics at molecule-metal interfaces plays a key role in various fields as surface photochemistry or the development of molecular electronic devices. The bare transfer process is often described in terms of tunneling through an interfacial barrier that depends on the distance of the excited electron to the metal substrate. However, a quantitative characterization of such potential barriers is still lacking. In the present time-resolved two-photon photoemission (2PPE) study of amorphous NH3 layers on Cu(111) we show that photoinjection of electrons is followed by charge solvation leading to the formation of a transient potential barrier at the interface that determines the ET to the substrate. We demonstrate that the electrons are localized at the ammonia-vacuum interface and that the ET rate depends exponentially on the NH3 layer thickness with inverse range parameters beta between 1.8 and 2.7 nm(-1). Systematic analysis of this time-resolved and layer thickness-dependent data finally enables the determination of the temporal evolution of the interfacial potential barrier using a simple model description. We find that the tunneling barrier forms after tau(E) = 180 fs and subsequently rises more than three times faster than the binding energy gain of the solvated electrons.