Long magnetic relaxation (up to few hours) between stable magnetic states was analyzed in Pt/Co/Ir/Co/ Pt/GaAs heterostructures of different Co layers thickness. The experimental data were compared to a large variety of theoretical models amongst which the Fatuzzo-Labrune one seems to be the more relevant. The contributions from domain nucleation and domain wall motion to magnetic relaxation of the spin valves were separated and evaluated. The increase of Co layer thickness suppresses the domain nucleation and enhances the domain wall propagation. The obtained data provide an understanding of the limitations of switching time in the spin valves of large area necessary for GMR biosensors.