Accrual income and expense

New client is on the accrual basis. There are some old outstanding invoices to clients (some form 2019) my client doesn't think he'll collect on. They were included in income. What is the time frame for writing these uncollectible invoices off as bad debt?

When it is determined they are not collectible.
Have they made any effort to collect?

When it is determined they are not collectible.
Have they made any effort to collect?

Thanks. I noticed it on the balance sheet. I'll check with client about collection efforts.