We have changed the way in which we provide voice mails and call recording files. In early February 2012 we moved from wav encoded files to mp3.
This change had several benefits; smaller files (5 times smaller compared to the wav file encoding), more playback options within the CRM system (jump to a specific point in the recording) and playback will be compatible with more mobile devices.
There is no fixed time for call recording storage.
All of your call recordings are automatically archived so that you can then search back at any time to retrieve a call recording as and when you need to.