you don't ever replace all the water in the tank. How much top off water (this would be fresh r/o water, not saltwater) you add depends on how much water evabortes. I add about a gallon every 2-3 days in the summer (75gl tank).
How often you do water changes depends on what you have. Some people do small ones weekly, some do bi-monthly, others monthly or less. There are many threads regarding water changes.
I would have water you get from the ocean tested, each time! Check for everything and be careful when adding to your tank if you already have fish/inverts. I don't know how much the water changes, but you'd want your levels to be the same when you did a water change.
I, myself, would not use ocean water.