Few points to remember:
- Create a custom formula field on the object with (Data Type - Formula & Formula Return Type - Number ).
- Use the formula: TODAY() - LastActivityDate
- Since "LastActivityDate" is a system field it will only be visible when inserting the formula.
- This field will calculate based upon the Last Activity field.
- Now you can create a Report on the object showing the field for Days since "Last Activity".
- Standard objects have the "Last Activity" field by default when you create a report.
- Last activity date for custom object will appear only when you have checked "Allow Activities" checkbox.
- Future activities can result in negative numbers being displayed.
- When you have Shared Activities for your Contacts, Last Activity will only show for the Primary Contact.
- Not all objects support "LastActivityDate" check the API Guide to see if your object does.