The development of localized fluidization is experimentally studied in the central plane of an immersed cohesionless granular bed using planar laser induced fluorescence and refractive index matching. The upward growth of the fluidized zone is characterized from the initial localized particle movement within a cavity to the fully fluidized state. The primary outcome of the present study is the identification of two very distinct regimes for the expansion of the fluidized cavity depending on the flow rate at the injection point: a regular regime that has been observed as in previous works, and a newly observed ultraslow regime that requires much longer time to achieve full fluidization. The ultraslow regime was formerly identified only in its nonstationary state as a cavity regime. Experimental results show that at a particular flow rate, the diameter of the injection port is a significant parameter in the evolution of the fluidization in the area close to the injection, while having almost no effect in the final phase of the expansion, provided that the granular bed is high enough. Consequently, the duration of the expansion from cavity to fluidized chimney depends strongly on the injection port size in the ultraslow regime, but only depends weakly in the regular regime. In addition, a parametric study of particle size, injection port diameter, and bed height is performed for the regular regime, based on an apparent divergence of the expansion time as the flow rate approaches a critical flow rate. For this regime, an empirical expression is developed that allows the collapse of the expansion time for all the different variables studied.