Calculating log size

[Poub]

I have a customer that today is just FW and IPS. They are moving off their Bluecoat and will use CP for APP/URL filtering and they will also moving to O365 (creating more traffic than today). Today they are generating 7G of logs per day and would like to upgrade their MGT for 6 months of log retention. The $1M question is how best to calculate the increase in log size once the APP/URL/O365 takes place.

Any guess would be appreciated (please show your work 🙂 

Thanks

[PhoneBoy]

Depends on how detailed the logs will be.
If you're doing Extended logs (I.e. Logging every URL accessed) and HTTPS Inspection enabled, I could easily see that number double.
However, there's not really a good way to estimate.