You can set the number of milli-seconds the router is considered to be reachable on this IPv6 interface between 0 and 3600000 (1 hour). A neighbor is determined to be reachable if positive confirmation has been received within the reachable interval that the forward path to the neighbor was functioning properly. If no confirmation is received within the reachable interval, it is assumed that the neighbor is unreachable.
This example sets the router reachability interval to 120000 ms (120 seconds) for VLAN 50:
System(su-config)->interface vlan 50 System(su-config-intf-vlan.0.50)->ipv6 nd reachable-time 120000 System(su-config-intf-vlan.0.50)->