You can see your current balance of Helix Credits on the Store menu. 

If you purchase Helix Credits from the in-game Store, your balance will be updated by the amount you purchased and the Credits will be available to you.

This shouldn't take more than a couple of seconds, but if it's not the case, please make sure that the transaction went through in the transaction history of your system:
If the transaction shows up in the history, please allow some time for the balance to be updated - in edge cases it can take up to 24 hours.

Of course, our teams are available if you have any questions or concerns.