From YPPedia

A shun prevents the target from being able to communicate with others (with exception to the /officer, /fofficer, and /royalty channels) for a certain length of time. Blackspots will shun a target for a fixed length of time (depending on the number of blackspots already received), while Ocean Masters can issue shuns of any length of time. Shuns by Ocean Masters do not affect the amount of time the next shun from a blackspot gives, but both types will end up on player records.

The time shuns are applied for is in log-in minutes. Logging off will cause the timer to stop running, and it will resume when the pirate logs back on.