Depends on the card:
SE up to 3 meters for less than 5 devices or 1.5 meters for 5-8 devices
for Ultra SCSI, 3 meters unrestricted (except for spacing requirement)
for Fast SCSI.
LVD up to 12 meters depending on bus rate and more than 2 devices,
up to 25 meters for point to point.
Oh wow, a retailer website.
And no 50 cm anywhere in sight.