Assuming a focal plane shutter and electronic flash, your camera's flash sync speed (or slower if you want some ambient light too). For the flash exposure, all that matters is that the whole shutter rather than just a slit is open when the flash goes off, which is an order if magnitude shorter than the shutter. In other words, the exposure is determined by aperture, flash power and distance - shutter speed doesn't matter as long as the shutter gets out of the way in time.