There's a SuiteAnswer for this, it looks like you guys are pretty close: https://netsuite.custhelp.com/app/answers/detail/a_id/36103/

Below is a sample Saved Search that will pull up Customers with no sales orders for the last 6 months. Users may just modify the date range depending on their search requirement.

1.    Navigate to Lists > Search > Saved Searches > New.

2.    Click Customer.

3.    Under Criteria tab > Standard sub tab, add the following filters:


-          Transaction Fields… > Type = Sales Order

-          Transaction Fields… > Main Line = True (Yes)

-          Inactive = False (No)

-          Date of Last Order = not after | relative | 6 | months ago

4.    Navigate to Results tab > Columns sub tab and add the following fields:

-          ID

-          Name

-          Date of First Order

-          Date of Last Order

5.    Set a Search Title.

6.    Hit Save & Run and check the results.