I bough my bought last April and the have optima batteries. I have had to replace the starting battery. The date is 4/11 on the batteries I have on my trolling motor. My question is when do you replace them? Do you wait till they are dead ( I don't want to be stuck on the lake with a battery) or do you bite the bullet and just get new ones every 3 to 4 years?

I normally replace all at the same time

Usually TM batts won't just die on you. They will slowly start to get weaker faster. For example, when they are new you might use your TM in a 15mph for 8 hours. Then after a year, under those same conditions, it might reduce down to 7. In 2 years, same conditions, it might only last 5-6 hours.

So whenever you feel like the batteries just have enough "umph" to them, then replace them. I replace all of my TM batteries at once.