I have a 2007 Marlin with twin 250s and I replaced all of mine in November,including the primer bulbs, but the reason I changed them is I found some sun damage to one of the fuel lines, plus I was headed on a 7 week cruise to the keys and did not want any issues. I asked my Yamaha mechanic, whom I trust, and he said that Yamaha does not have a stated replacement period, but you should look for cracking in the lines, etc, or any sun damage and if you find any, replace them, and he said to do both engines even if you don't see any on the other one.