I doubt the "average" would be worth much, as there are too many variables to be useful. To get 180 days at sea, you likely need to be posted to a ship that is doing routine sailing for a couple of years, or get posted to a ship doing an OP Horizon and get it in under a year.
When I went to the coast I ended up deployed for 8.5 months as soon as I arrived, so I had my 180 in no time. Back then, it was 365 for the gunmetal, so I still waited a couple of years.