Phone status is held inside of the RIS DB (Real Time Information DataBase).
The information in this table has a shelf life of only like 48 or 72 hours. So, if the phone in CUCM shows the ip address, and says unregistered, it was unregistered with the last 72 hours (or so). If it has no IP address and says unknown, then it was atleast 72 hours ago (or so), if not longer, and then there's not way to tell, unless you archive your log files off box and can search through gigbytes and gigbytes of log files.
Alternatively, you can turn to CDR, and ask this question "If a phone hasn't received or placed a call in the last 90 days, can I delete it?"