Stitch length IMHO depends on material, thread size, personal preference, style and your local markets preference. I.e. a .8 mm thick thread or 1 mm even needs a longer length than a .5 mm thick thread and so on. 8 spi on a saddle looks not so great as it might on a watch strap. Same goes for a Celtic or Norsk item versus an Italian designer belt...
For wallets I like around 3 mm long stitches and I'm using a tera 20 thread. On bags I prefer a longer stitch of 4 to 5 mm depending on the leather I'm using.
This being said, the conversion of metric stitch length to spi is as follows: 25.4 / X = Y and will result in
8 spi = 3,2 mm
7 spi = 3,6 mm
6 spi = 4,2 mm
5 spi = 5,0 mm
(source: https://tv-sewingcenter.com/sewing/metric-conversion-from-us-stitches-per-inch)