Actually this depends on how many alerts are coming today and how much it is expected to increase over time. You can check the per day size of vunet-1-1-notification-xxxx.xx.xx indices for the last X days you are currently retaining and check the max size consumed on a day. Then you can try to use this kind of calculations
Max size consumed per day (Only primary shards) = 30MB
Add about 30% buffer =~ 50MB
Size Per Month you need on /data disks = 1.5 GB
In case you need replication (only if you have multiple analyzers), you need double the space (~3GB)
For offline archival space, Lets assume 20% month on month increase in alerts, Total size you need for 6 months is approx 9GB of space in any offline archive partition. Only primary shard space is considered for archives.
Hope this helps.