This is a simple steam boiler maintenance that I would recommend to do once a year. If not every two years if you can get away with it. Its important to understand when dealing with these things that fresh water is the enemy of the boiler. Water that has rust in it has almost no potential to continue rusting the metal so dirty water if it hasn’t become sludge is actually good. If your water seems to always be clear you may want to assess if there are any steam leaks in you system. Common ones are valve packings and vents on radiators. If it’s possible at all for you to monitor the boilers water instead of just letting it fill when it gets low I would highly recommend that as well
