This study evaluates the influence of four geosynthetic stiffness characterization approaches on the deformations of geosynthetic reinforced soil (GRS) abutments, including (1) a hyperbolic model considering the cumulative creep duration during staged construction; (2) a hyperbolic model with a uniform 1,000-h creep duration; (3) a linear elastic model using constant stiffness from constant rate-of-strain (CRS) tests at 2% strain; and (4) a linear elastic model using constant stiffness based on 1,000-h constant load creep (CLC) tests at 2% strain. Numerical simulations demonstrate that the characterization of geosynthetic stiffness significantly affects footing settlements and facing displacements, with the influence increasing under higher vertical stresses. The results indicate that using stiffness values from CRS tests which neglect time-dependent creep, leads to underestimation of abutment deformations. Assuming a uniform creep duration across reinforcement layers tends to overestimate creep durations for most layers, thereby exaggerating predicted deformations. In contrast, the cumulative creep duration approach during staged construction, which accounts for the varying creep durations associated with different construction stages, yields smaller deformations. Additionally, applying a single-value geosynthetic stiffness from CLC tests results in slight overestimation of deformations.