How often do VPN's change your IP address?

My first introduction to the concept of a VPN was when a few people in a forum were discussing how often their IP address changes while they’re online. I remember one of the people involved said his changes 30 times per second.

I have no idea how legitimate that is and my google-fu has failed me, likely due to not understanding the technical terminology.

How often do normal VPN’s change IP addresses of their users?