When does an account become uncollectible?

"Bad debt expense, or noncollectable accounts expense, or doubtful accounts expense. When does an account or a note become noncollectable? There is no general rule for determining when an account is noncollectable. once a receivable is past due, a company should first notify the customer and try to collect the account. if after repeated attempts the customer doesn't pay, the company may turn the account over to a collection agency. After the collection agency attempts collection, any remaining balance in the account is considered worthless."

-Principals of Accounting book, page 394-