Most likely you will never have a scratch on your space black DLC coating. I have had three SBSS apple watches and they all still look like new after years of wear.
It’s less durable than the DLC coating on the space black stainless, but the PVD (Physical Vapor Deposition) coating seemingly from what I read, has been fairly durable in terms of deflecting against minor scratches. Also, the goal stainless bands on the Gold iPhone XS has also had good reports of resisting scratches, as they both use the same type of coating.
Similar thread discussing the PVD coating on the gold iPhone XS: