The livespan of bottom paint greatly depends on where you sail, how thick an application you applied, the type of paint used, where it was thinned, whether your boat is in the water year round or hauled for the winter, and how often you have your bottom cleaned. In most locations you cannot expect to get more than two years out of bottom paint. Most harder bottom paints will only last a year. There are some ablatives, that if applied thick enough and kept ''fresh'' will last for more than two seasons but they become so ineffective by the thrid year that they are not really functioning as bottom paint. Some hard racing paints are the same way. I got three years out of Baltiplate but I had a diver every other week in summer.
Beyond that It is not a great idea to leave a boat in the water that long as it will uptake some water even with a barrier coat. I am always amazed at how high my boat floats after she has been out of the water even for 2 or 3 week.
Jeff