The change in microstructure in a long-period stacking-ordered (LPSO) phases containing Mg-RE alloy by longer solution heat treatment time, up to 120h, after hot extrusion is of interest in this study. Previous research has shown the initial fine-grained microstructure, consisting of grain boundary blocky and lamellar LPSO structures within the matrix, changes into coarser grains of less lamellar and blocky LPSO phases when a solution heat treatment time up to 72h was applied. Exceeding this time up to 120h causes a reduction in grain size and an increase of lamellar LPSO phases. The visible changes in microstructure are accompanied by nanohardness measurements to analyze the nanomechanical properties of the phase transformation. Blocky LPSO phases show a higher hardness than the grains with or without lamellar LPSO phases, and their hardness decreases with heat treatment time. The hardness of the Mg-matrix depends strongly on the surrounding phases, in between LPSO lamellae the hardness values are found to be the lowest. It was possible to correlate the hardness to solid solution strengthening in dependence of the appearance of blocky and lamellar LPSO phases.