I don't imagine an average LMB getting longer than 18-24" in a home aquarium. Since so few people actually keep them for the duration of their lives its difficult to know for sure, due to a lack of information. I prefer to use tanks that are 4X the fish's length and double their length in width. So for a 2' LMB I would suggest something along the lines of a 8' long 4' wide tank. If the tank was 2' high it would make the tank 478g. Some people will stock a tank with a fish that reaches the same length as the tank's width. If said tank were 2' instead of 4' wide the tank would be 239g. Thats of course assuming the LMB actually reached 2'.