You can set the interval between neighbor solicitation messages in milli-seconds between 1000 and 4294967295.
This example sets the interval between neighbor solicitation messages at 1.5 seconds for VLAN 50:
System(su-config)->interface vlan 50 System(su-config-intf-vlan.0.50)->ipv6 nd ns-interval 1500 System(su-config-intf-vlan.0.50)->