How one can set the Due In Counter Days?

Letter Due In Counter:  
Once the round gets completed i.e. printing and downloading the letters is done, the system automatically activates the next cycle after which the report from the monitoring agencies has to be re-imported. 
This value of Letter Due In Counter can be set at Settings -> Credit -> Letter due in section. 

This value set for the Letter Due In Counter will be reflected on each individual clients record w.r.t the date on which the round id completed. 

This value can also be seen provided at the Clients listing page in form of a column named 'Next Import'; this column can be sorted which will help you to list the records in the desired sorted manner.