cTrader Bonuses are used in conjunction with Balance by adding it to the Equity of the account to open positions. Bonuses can not be used instead of the Balance. If an account's Balance is zero, then so will be the amount of the Active Bonus.
Once a Bonus has been given, it can not be taken away and can not expire. The only way the Total Bonus can be reduced is if it is converted into the Balance. Your Broker will set the criteria for converting Bonus into the Balance. If a Broker does not allow their Bonuses to be converted, then the Total Bonus will remain inside the account indefinitely.
At the end of each day, at midnight UTC Bonuses will be converted if the necessary criteria have met. Once a Bonus is converted, it will be added to the account’s Balance as a deposit and can be withdrawn. The amount of Bonus which was converted will be withdrawn from the accounts Available Bonus.
As the Bonuses are added to the Equity, they can be used to open positions and avoid reaching the stop out. As the Active Bonus can not exceed 100% of the accounts Balance and Unrealized Net P&L, this would mean that if the account Balance is zero so will Active Bonus.