Let's suppose I'm drawing up a list of characteristics that will help me in deciding which books I'll get rid of (not that I'm really going to do that, it's just a made-up example).
We will consider those books that:
- have green covers
- are paperback editions
- have been read more than a certain amount of time ago (the exact period of time will be defined later.)
Is the expression more than a certain amount of time ago correct? What I mean here is that at the moment, we still haven't agreed upon an exact criterion (3 months, 2 years, whatever). It's like saying "more than X [days/months/years] ago".