Have you read the Help section? It has some useful stuff.
Be fair and honest with your feedback. And take a little time to write a detailed comment along with your feedback ratings—this will help future clients and freelancers learn from your experience.
The system is double-blind, which means that your feedback won’t show up until both you and your client provide feedback for each other. The feedback period is 14 days from the end of the contract. If only one party leaves feedback, it will show after the 14-day feedback period has expired. Please keep in mind that it takes 12 to 24 hours for your public profile to show your feedback after both parties submit or after the feedback period has expired.
If you are the one who ends the contract, you will be prompted to leave your feedback for the client immediately after you click the End contract button. If you did not end the contract, your client will provide feedback first, and you should reciprocate as soon as possible. If you don’t leave feedback, your contract with them will be excluded from their feedback calculations but will not affect your own.