Silane-functionalized silica (SFS) materials are widely used in catalysis, chromatography, filtration, and energy conversion, yet their long-term stability remains poorly understood despite extensive technological relevance. Here, we report the first experimental assessment of the decadal (20-year) stability of grafted silica (C8-SFS) under different storage conditions, including atmospheric exposure and continuous contact with water under repeated high-pressure intrusion–extrusion cycling up to 30 MPa. Remarkably, all aged samples preserved their structural integrity and hydrophobic character, showing only a minor decrease in intrusion pressure and in intrusion–extrusion hysteresis compared to pristine silica. Surprisingly, the sample hermetically encapsulated with water under high-pressure cycling performed slightly better than the atmosphere-exposed sample. Thermogravimetric analysis confirmed the persistence of covalently bonded silanes, while SEM imaging revealed no particle fracture even after hundreds of compression cycles. Slight reductions in extrusion pressure correlated with marginal increases in silane mass loss, consistent with minor hydrolysis or conformational rearrangement of grafted chains. These results demonstrate strong chemical and mechanical durability of C8-SFS, highlighting that water exposure under moderate temperature and neutral pH does not compromise the intrusion–extrusion performances of the material over decades. The findings have direct implications for the long-term reliability of silica-based materials in chromatographic, damping, and energy storage systems.