Scientists generally hold that language has been so long in use that the length of time writing is known to cover is trifling in comparison.

that the length of time writing is known to cover is trifling in comparison.为结果状语从句，呼应so long. 其中the length of time为主语，writing is known to cover为省略了关系代词that的定语从句，that指代先行词the length of time, 并在从句中作to cover的宾语。

翻译：科学家们普遍认为，语言应用（口头交流）的历史如此之久，以致于相比之下已知的书写应用（书面交流）的历史是微不足道的。