Answer: Upon completion of the sale or when services have been performed
Explanation: Revenue is recorded in the books of accounts when the obligation from the seller side has been done and he receives the right to collect the charge of service as per the contract.
In general,the obligation is considered to be satisfied when the goods or services are received by the purchaser. Then the seller gets the right to ask money and can record revenue in books as per the accrual basis of accounting.