This study investigates the time-dependent rheological behaviour of cement mortars formulated for 3D Concrete Printing (3DCP) with a focus on both direct and indirect methods for assessing static yield stress. Three experimental methods were used: two direct methods—(1) parallel plate rheometer and (2) shear vane rheometer—and one indirect method—(3) the mini slump test. Four mortar mixes with varying water-to-cement ratios were tested. The study analyses discrepancies among static yield stress values obtained from different methods and identifies the most suitable one for 3DCP applications. A comparative analysis was carried out to relate the measured slump values with static yield stress, leading to the development of an empirical model. The model was validated through a series of 3D printing experiments. The results reveal an exponential correlation between slump and static yield stress, influenced significantly by the surface area-to-volume ratio (A/V) of test specimens. A generalised exponential expression is proposed to estimate static yield stress based on the measured slump and A/V ratio, which can be used to predict the maximum number of printable layers without plastic collapse. The empirical models have shown good agreement with experimental data, suggesting their practical utility in predicting printability based on time-dependent slump values.