Because of the Active/Passive HA configuration, all active sessions will break and need to be re-established once the Passive unit becomes Active. Total fail-over time is typically 10-15 seconds max.
You can also refer to RFC 3768 for more information. The VRRP router that is master will respond to LAN hosts with a virtual MAC address. If the slave becomes master it will initiate the WAN connections and sessions can be re-established for running applications.