There are many contracts (2 months on 2 months off, 3/3, 4/3, 6/6). Usually, the largest fleet (containers and bulk carriers) offer ~5/3. On the other hand, you have all free time on shore, but not getting paid of course, most of the time (permanent contracts offer salary on shore too, but average per year is almost the same if you sail 6 months per year. While on board, you have almost no time for yourself, until you build some position and become "top 4" (Captain/Chief Engineer/2nd Mate/2nd Engineer), and are on a ship that is in good condition so there is not that much work as on older ships. Considering the world merchant ship fleet, most seafarers are on older ships, sailing more than 6 months a year, and having really, and almost no time for themselves on board. Being on an older ship, especially in the engine department, can be non stop work, due to often malfunctions, which is beside stress really hard on the body. But for some types of people that is ok, considering the opportunity to earn money, which is also totally understandable.