How often should I be changing these plugs. It a 2007 mercury pro xs 250.

Mercury says to replace after the first 100 hours and inspect annually after that. The 0.6 mm iridium tip will wear and when you notice that the top of the tip is slanted and no longer flat I would replace them. Carbon also likes to build up and can foul them. Keep them clean at least.

And they are 8- 12 dollars at napa or auto zone ect and 21-24 dollas a piece at a marine dealership