According to this thread, the limit is 10 messages, 5 minutes maximum each, stored for 10 days. Interesting that these details are not covered in the knowledge base. @Alan_K, please update the voice mail knowledge base with these limits when convenient.
Your voicemail box can have 10 messages, whether new or saved. Each message can be up to 5 minutes in duration. After 10 days, a message, whether new or saved, is automatically deleted, unless saved
