"Download copertina" is probably "Downloading covers". That is a step in the usual sync and should happen during the account repair.
How long it takes depends on what is wrong. If everything was OK, it can take a few minutes. The device compares what it has with what the server thinks and gets what is needed. So, if there are a lot of problems, it will take a while. But, 16 hours seems to long unless everything had to be downloaded and your connection was slow. Has it gone to sleep at any time? I can't remember if this blocks the sleep or not. If it does, that will add time.
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