Regarding bill books and expenses files

For income tax assessments. You need records for that. Gst law prescribes 5 years.

And in today's digital age keep all your data in digital formats. Easy to store and handle

8 years is prescribed as per law and as per gst that should be 5 years

72 months ( 6 yeras ) from  the due date of furnishing of annual  return for the  year  pertaining to such  accounts and  records . 

if you were an assessee in VAT regime and any of your assessments are pending. then due date is 5 years from such assessments.