Photorefractive gratings are generated in a BSO crystal by the interference of two 25-psec pulses at 532 nm. The grating is analyzed by diffraction of a third pulse or a weak cw argon laser, which permits a continuous monitoring of its evolution from picoseconds to milliseconds. The grating builds up in two steps; their relative contributions vary strongly with fluence. This behavior is explained by means of a band transport model with shallow traps. Values of mobility, lifetimes, and diffusion lengths of carriers are derived and compared with those obtained in experiments conducted with low-power lasers; the values show a striking continuity from cw to picosecond regimes.