Radius stale sessions

Hello everyone, I am fine-tuning the radius accountig sql information and noticed that there are multiple stale sessions. I would like to use a script in cron to delete these sessions but I have a question. How should I select a session as stale? Time difference (acctstarttime and now()) ? Other records with the same mac-address and open sessions (acctstoptime is null) ? Regards, Renato L. Sousa