It's great that you're looking to keep you list clean! It makes your response numbers looking good and helps clean our people's inbox who aren't opening anyway!
So first to your question, others can certainly chime in here, but I feel like 18 months is a good place to start. I only pull that number from our import policy stating that you must have done business with them in the past 18 months to send mail to them. Just seems logical to me.
Others I've spoken with have set up segments looking for folks who haven't opened in the past 6 months, which is definitely the more extreme end of the scale.
If you need help creating those segments for cleaning house, check out these articles on our resource center: