The quota is the amount of mentions you are able to receive every 30 days from the day you subscribe. Every mention received on your feed is deducted from your quota.
Once a mention is collected, it stays available in your account and can be available even if you move it to spam or trash when you are under a paid plan.
Free plans are offered 30 days of archive.
But be careful!
Removing or deleting content from your feed does not give you back quota. While your quota is over, the platform will stop receiving crawling for matching content. You will need to wait for the next renewal or upgrade your plan with a larger quota limit.
So make sure to set-up alerts that match with your capacities :)