Victoria, we keep our known, but inactive or dead lead records for at least 2 years to facilitate year-over-year performance reporting. After that, they are a candidate for deletion.
We never delete blacklisted leads to ensure we never send them emails (there are very few of these anyway).
Email Invalid is a strong indicator that someone has left their company and we have our teleprospecting or sales team follow up on these to determine if this is the case. If so, we change the Lead Status to "Gone" and try to identify a new lead at that company. These are candidates for deletion after 2 years.
If someone unsubscribes, they may still be receptive to calls, coming to events / tradeshows, etc, so we do not flag those as dead or bogus.