Wow! 59¢ is a good price. I think BPS is something like a buck and a half. I usually buy 4. Good thing....when I did it this year I couldn't find the ones that I just bought, but the extra ones I bought last year was hanging on the pegboard.
You can change it every six months or as often as you want. I change every six months a couple of times after I saw signs of water in my oil. I never saw the water again so I went back to yearly. You'll probably be busy with other maintenance projects on your boat to want to do the LU service more often than you need to.
What kind of boat do you have?