I have had leathers skin over and remain unopened for almost 2 weeks. So long that they had slight algae growing on the skinning or sloughing layer. Not in many many years so I must be doing something better with my water in the last 15 or so years.
If it has areas that change color other than an opaque skin kinda like snakes shedding then may want to do a smell test. Healthy leathers have a distinct scent but usually only present when cutting them with scissors or razer blades when fragging. However, if it stinks of dead clams it may be in trouble.
I have also had portions rot due to falling behind something or something falling on it and have used a powerhead to clean off the bad stuff and with good water parameters/not being too far gone, the leather has usually recovered.
Since I have bought about 4 or 5 leathers from Dave Burgman over the many moons he is of course an excellent resource.
I'd say hey to Dave and all but I don't want to hijack the thread. hmmm I guess I just did.
Anyway, good luck with your leathers.