What's the minimum exposure time that can be achieved in bulb mode?

1. Technically, the minimum exposure time is probably limited by the speed that a person can press and release the shutter button (or remote shutter release). I assume this is somewhere on the order of 0.1 seconds (1/10 shutter speed) or so. However, this is highly variable and difficult to reproduce shot-to-shot, even for the same person using the same equipment.

2. Practically, for exposures over a second or two, fractions of a second in exposure don't really matter. For instance, a 2.5 s exposure is only 1/3 stop more exposure than 2 s, and 5.5 s is less than 1/7 stop more exposure than 5 s. When bulb mode is necessary (i.e., when you need exposures longer than 30 seconds), even 1 second resolution is immaterial: 31 seconds is a mere 1/21 stop more exposure than 30 seconds.

Is there a way to retrieve the actual exposure time in either the metadata or elsewhere?

I use Nikon, but the "Exposure time" EXIF data in my long exposure shots shows 0.1 s resolution. Canon cameras round reported bulb exposure time to the nearest second, even if the actual exposure time is a fraction of one second. I have searched Flickr for Canon 1200d photos with long/bulb exposures, but of the few that had "Bulb duration", only integer seconds were shown.