The basic characteristics for breakdown of liquid dielectrics are observed in this work on the grounds of ideas about the development of electro-hydro-dynamic instabilities. In particular, the main attention is paid to discharge time water, which dielectric properties change in dependence on electrical conductivity. It is shown that time of discharge lag is divided into two main periods: time of discharge lightning and time to form conducting channel. Both time intervals are defined by electro-hydro-dynamic instability. In case when electrical conductivity of medium increased, the mechanism of overheat instability, which gives the additive contribution to the process development, is observed. The analytical expressions for time of discharge lightning and velocity of discharge distribution in aqueous medium are obtained. The comparison with experimental data gives, in our opinion, rather satisfactory coincidence of calculated and empirical data both for conducting and non-conducting liquids.