我写了好久用英语怎么说

这个说法可以用"I have been writing for a long time"来表达。"I have been writing"表示"我一直在写"，"for a long time"表示"很长时间"。例如：
I have been writing for hours. (我写了好几个小时了。)
I have been writing this essay for days. (我已经写了这篇文章好几天了。)
希望这些信息对您有帮助。