We would like to write off all WIP that falls into 120 days + is it possible to use Auto Fees to accomplish this?
Yes, Auto fees is the best way to go to achieve this.
For the WIP Date they will have to use the Exclude WIP data after field.
In this case by calculating the 120 day "barrier" and adjusting the date in this field to suit.
The entries created are WIP Adjustment Only transactions and will show in Fee Spy but not in the Debt Console of Client console.
As the fees will be created in a batch they can be Posted and Cleared as a batch.