EV Storage expiry

We want to turn on Storage expiry and need to estimate how long it will take to delete the expired items.

We have about 15 Million items to delete from 3851 users totalling 1.5TB in size.

Does anyone have experience of storage expiry throughput rates and could they estimate how long this would take.

If we ran storage expiry 22 hours per day with no auto-archiving - how many days would be required?.

Environment is EV 9 on Win 2008 server.

thanks