When to ditch a GB that you no longer need is purely situational, your situation, not anybody elses. The LOA provides a supply multiplier to your non-GB buildings, and goods, either relevant at age goods or utterly irrelevant goods from 3 ages prior. I think you hit the 'worthless goods' payout in modern era. Personally I got rid of my LOA in Contemporary Era a couple years ago, though if I had to play the game over again I'd never bother building that GB. And if you do any GbG you'll probably find you get far more goods from that than from a building like LoA. That goes back to the 'situational' comment. Lots of ways to make goods and supplies in this game, many of which render what was a great GB 4 years ago a trash one today for a lot of players.