Default VM transfer is at 15 seconds I believe. The range AT&T allows is 15-30. You can't change that time from the iPhone (another basic phone feature they chose to exclude) but it can be changed via GSM codes or by pitong the SIM in another phone that can change it. I have mine set to 25.
Verizon iPhone 6 128GB, T-Mobile (free 200MB) iPad Mini 2 Retina