In the equilibrium time of scour prediction, it is important to know how to calculate the equilibrium depth of scour, in order to predict the depth of bridge foundations (calculate how deep the foundations need to be in the river bed), for example, to appoint the foundation pile length of engineering-structures in river flow. The first problem to be faced is to define such a time, and consequently the equilibrium scour depth in clear water conditions, because the scouring process will never cease completely. Proposed threshold criterions known from the literature for equilibrium time of scour are only depending on the size of the bridge structures, and not considering hydraulic parameters of the flow. Based on differential equation of the bed sediment movement for clear water conditions, new formula for time of scour determination was elaborated and new hydraulic threshold criterion was proposed. In this study an analysis of the hydraulic and river bed parameters that have an impact on the equilibrium time of scour at clear-water conditions in plain rivers was also made. It was found that the equilibrium time of scour is depending on the following parameters: contraction rate of the flow, Froude number, bed layering, grain size diameter, local flow velocity near the structure, ratio of local velocity to critical one. The test results of scour evaluation in time with duration of 7 hours (Gjunsburgs & Neilands, 2004) were prolonged till the equilibrium stage of scour and compared with the equilibrium time of scour calculated by the method presented, and they were in good agreement.