There is no limit on the amount that can be written off by the Company since it depends upon the decision taken by the management. However as an auditor, we need to see the reason behind such written off. If the amount is material enough and is a continuos one, then we should look into the Going concern of the Company. Also, we need to verify whether any subsequent sales has been made to same parties whose balances are written off and obtain reason as to such decision. Hope this clarifies ur query.