All bonuses and winnings with free spins on Casino Heroes have a wagering requirement of x40 (if nothing else is stated). This means that you need to play for the amount 40 times with your bonus money before it is converted into real money that you can withdraw.
To see your personal wagering balance, click on your account balance and navigate to the Withdrawal page. There you will receive a message that tells you your current real money, bonus, and wagering balance.
Please note that the maximum bet allowed per round with bonus money is 5€, any bets higher than that will be declined.