Can it be used from any country? And do the credits expire after some time?
check this out -
https://console.developers.google.com/freetrial
to know all details directly from vendor.
Credit limit for 60 days.
It should work for any Google Cloud Platform account that is active (ie use credit card or bank to verify. I highly suggest virtual credit card if you have access. If not some sell them on digital goods.)
If you can prove some hole in system and code did not work you will be given refund minus transaction fees to send.
New $300 credit limit is 60 day's. The additional $500 is 90 day's. You can see in second picture the months on a recently signed up account.