I have had a 2013 MBA since last October (i.e. about 7 months), it has a 512GB Samsung SSD and I use it daily and quite heavily (it's my primary and only computer). I used DriveDX to check the SSD's health and it reports that its "wear leveling count" is at 90% (see attachment). Is it a normal value? Does it mean I am 10% into the drive's lifecycle, or something worse or better?
Thank you very much.
Only the people creating that utility might know, and even they probably don't
You have your Time Machine backup (you have a Time Machine backup, right? ), so if the SSD drive breaks within 12 months it will be fixed under warranty, and you have 12 months time to buy AppleCare which will cover it for three years, so it will be fixed for free for three years. And having a Time Machine backup, you just restore your backup, and everything is fine.
Would this mean that you now have about 115GB useful left? (90% of 128GB)
No.
On an SSD drive, every 128KB block can be written a certain number of times, say 100 times (I make up this number, I have no idea what the actual number is). Now you have blocks that are written a lot, and others are never written at all. Say you have a 100 MB music album, those 100 MB will be written once and never be written again as long as you keep the music.
When the SSD drive notices that some blocks are written a lot (say 90 times) it swaps them with blocks that are used very little. So your music album moves to a part of the drive that has already been written 90 times, but that's Ok because it is probably not going to be written again. And the data that was written again and again is now in a fresh area with only one or two writes so far.
So you have all the 128 GB available, until suddenly everything reaches the write limit and breaks down. Like your car, which doesn't slow down when the tank is half empty. When the tank is empty, it stops. Until then, it drives just fine.