It's been a long time since I started to teach at this school.
我在这所学校任教已有很长时间了.
【说明】It's = It has + been 因为后面的时间状语从句since引导,主句用完成时态.填quite 为什么quite adv.相当, 颇, 有点儿的意思这里用quite a long time也可以，是固定搭配，用于强调时间很长。eg： I had waited for quite a long time before he turned up.我等了他半天，他才来。