For sample drives, there is no reason to leave ANY spare space.
The spare space was only to allow there to be a big enough pool of spare blocks so that write exhaustion (10,000 writes per block) did not occur during the likely life of the drive.
For hobbyist’s DAW use as a project drive, as I enumerated in this Re: Buying a new PC - advise please thread post, an SSD is likely to last about 100 years before ANY blocks become unwritable with only 15GB of spare space. For a professional studio, they should get over three years with only 100GB spare.
Those were likely worst case figures, but the drive would probably suffer a catastrophic electrical failure before write limits were reached.
Note that the non-loss lifetime is directly proportional to the space capacity for a given usage pattern, but you can put your own usage values into the formulae to derive how much spare capacity you need to leave on an SSD.