We would store all data and attachments without time limits and we wouldn’t delete any data automatically. The storage limit is based on a fair use policy, which means that you can store as many attachments per user as long as it’s reasonable (i.e. not uploading many videos per user per day). If you are using TestRail in a typical way you wouldn’t hit any limits and so far (with many thousand users and teams using TestRail), we never had to limit storage space or similar.
If you have specific requirements etc. you can also contact us in advance. For the API we throttle requests if you send many concurrent requests to protect your account’s performance and the API would signal this so you can repeat the request. But again this would not be an issue with normal usage.
I hope this helps!