Hi Jacqueline, 


Description    
We can follow the below-mentioned steps if we need to show a Field on the record or a Report with information with Days since "Last Activity".

Resolution    
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".
​Few points to remember:
Standard objects have the "Last Activity" field by default when you create a report.
Last activity date for the 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.


I hope you find the above solution helpful. If it does, please mark as Best Answer to help others too.

Thanks and Regards,
Deepali Kulshrestha