In our earlier papers [M. Rak, Proc. SPIE 3178 (1997) 108 and M. Rak et al., J. Cryst. Growth 197 (1999) 944 ], we solved the time-dependent equation of Burton, Cabrera and Frank (BCF) for transient state conditions occurring immediately after submersion of a crystal surface in supersaturated solution. As a result, the expression for the transient surface supersaturation was found and discussed. In this paper we present an approximation of that expression. The approximation enables a simple estimation of the time required to attain the steady-state value of the surface supersaturation.