All services that offer Free Quota have limits on what you can use without being charged. Some Free Quotas will be resetted on a monthly basis, some will be provided once and can be used until you reach the limit of the provided Free Quota. Each service has a different type of limits that is unique to that specific service.
Some of the most common types are limits by time (in that case it's tracked by minutes) or by transferred or stored data (which would be tracked by megabytes / gigabytes).
It’s important to know, that all quotas we refer to in our Help Center, are account-bound. Let’s say for example you have a Free Quota of 50GB storage. If you’re using 10GB storage in project A and 5GB in project B, you used 15GB storage of your Free Quota.