In the store book of Charterhouse School, Godlaming is an entry under the year 1618: mending of pottes and pannes with waxing clensing and colouring of Jackes xiiiis. (Baker, p73)
While nicely justifying my use of beeswax, it refers to the practice of recolouring black jacks. Jacks, generally being black, were coloured with with a particular type of dye that contains a quantity of iron in solution. The iron reacts with the tannin in the leather, forming ferric tannate. There are any number of recipes around covering a wide range of preiods. Some start with oak gall in an iron pot , others with iron scale or even hot iron in an acidic solution (Theopolus, Bk1, Ch38). It tends to be a very dark blue-grey, which darkens to black on
application of wax, tallow or other forms of fat. For a comparison see What Tallow Does to Dyed Leather. So how often would it need to be redyed, or would it just need retouching on the scrapes and rewaxing be sufficient for the rest?
Some years ago, I was attaching a leather rim to a shield and left an offcut outside with a few iron tacks sitting on top. The following weekend, after a few showers of rain (yes, kids it used to rain in Australia once), I discovered the leather fairly crisp from having all the remaining fat washed out and a strong black colour. A few friends and I had been having a pseudo-argument about colourfastness of medieval dyes in the letters pages of the magazine New Hedeby. Never one to miss an opportunity to do some extreme testing, I cut the black leather in half, nailed one half to the fence and sent the other to one of these friends who, at the time, was living in Port Hedland. They keep the sun turned up to 11 all the time in Port Hedland. He also attached his to a post in full sun.
The results? Mine was still black when about 10 months later the fence blew over. His continued to be black until two years later, when the leather finally crumbled to dust.