Unfortunately there’s no one answer for that. There are many factors that determine the right answer. Some of the factors are: type of bottom paint, lake temperature, lake nutrition levels, amount the boat is moved, if the boat is ever dived on to clean it in the water, tolerance for slime build-up vs needing a perfectly smooth bottom, etc. Your profile says you’re in Texas so I assume you’re not planning to stay on the hard each winter. If that’s the case I’d say 2 years as a rough starting point, to be modulated by all the answers to the above.