Hello,

In the Merchant Navy if you want to progress to be a Captain of the ship then you have to give a competency exam at each stage, the qualifying aspect of giving specified exam and acquiring the specified sea time.As a Deck Officer you can appear for the exam after having 18 months experience as Chief Officer. Once you clear Master exam, you can be promoted to Captain Rank by your company.
And generally it takes 10-11 years to become a Captain.

Hope it helps
Thank You and Good Luck !!!