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There is no danger of curing to long (Within reason), only the longer it's in the cure the saltier it will get. try to go by the stated directions or as close to it as possible.
I agree with others below about the time but I don't think ground meat would get any saltier.
Getting saltier over time applies to a chunk of meat in a solution or dry rub that will get saltier as the salt and cure gets farther into the meat via diffusion. In a mix the salt/cure is added and is constant regardless of if it makes it into the meat cells or not.
In other words you add some amount of salt and none of it will get left behind. If you pull meat out of a brine (or rinse off a rub) some of the salt gets left back in the brine.