said by GMAIL FAQ:One gigabyte of storage means the average user can go five years without ever deleting a single message, so we don't expect you to get close to your quota anytime soon.
However, if your mailbox nears its storage quota, you'll see a warning message at the top of your inbox letting you know how much storage you have left. If you reach more than 90 percent of your storage quota, we'll also send a message to your alternate email address (if you've provided one) to let you know.
If you manage to go over your storage limit, Gmail will hold your messages for 10 days until you clear out some of your old messages to make way for new ones. If you don't delete anything within the allotted time, you won't be able to receive new messages and messages sent to your account will be returned to the sender