There is an interesting story in Song dynasty that a farmer was long to catch rabbits by waiting nearby a big stump after he once unexpectedly caught a rabbit that had run into the big stump out of nowhere, broken its neck and died instantly. After a year of sitting there, the farmer hasn't seized a single rabbit and grass has grown all over the ground. I think this is longest you can get. You can't expand the size of a sentence simply by adding the conjunction 'but' when its meaning is complete. 个人觉得英语里非常注重句子的逻辑关系，如果一个句子的逻辑关系已经完整，就一定要句号，强行用连接词把不存在逻辑关系的句子接上，反而会让你的分数更低（考虑你应该是作文需要）。