You get a 5-day grace period. When it expires, your account will turn into Basic. In other words, your space will be downgraded to the currently available for free.
Any files beyond this free quota will remain in your account for 30 days. After that, files exceeding your account storage space will be moved to the Trash on a random basis. The deleted files will be kept in the Trash for another 15 days. During this time, you will be able to upgrade your plan and retrieve the files.