If 51% attack is successful, the owner of the dominant hashrate have the power to reverse any transaction, it all depends on the hashrate.
You can read this which answers your question:
https://bitcointalk.org/index.php?topic=5382044.msg59063011#msg59063011 Here's some noteworthy results:
- With 30% of the hash rate, you have a ~62% chance to reverse a transaction with 1 confirmation.
- With 40% of the hash rate, you have a ~55% chance to reverse a transaction with 5 confirmation.
- With 20% of the hash rate, you have a ~10% chance to reverse a transaction with 3 confirmation.