how much time a buyer will get refund after giving 5 start review and accepting the order ?
“Completed orders (or, where applicable, completed milestones) may be cancelled, upon review of our Customer Support team, up to 14 days after the order (or Milestone) is marked as complete.”
The above is from the Terms of Service.
Suggest you read the Terms of Service at the bottom of the Fiverr main page for the rules as to how Fiverr works.
Everything is explained in detail there.