Some of the oldest and most common non-destructive tests used in the industrial practice are liquid-penetrant inspections. They enable, however, the detection of surfacing discontinuities only. Nevertheless, the possibility of estimating the size of these discontinuities may be useful in the technical diagnostics of products subject to inspections. This inspired the work aimed to determine the dependence between the size of an indication and the time of its development in the function of the size of a discontinuity. This article contains research results, their analysis and final conclusions.