We never drop keys unless the expire time is set. As long as someone isn't abusing the service by flooding it with random data, it will scale for them.
My understanding is that one of your main ideas is to encrypt the data so I am honestly wondering where you would even start to check if a user was randomly flooding you or legitimately storing lots of keys.
Not trying to pick nits here... My original point was that -- in my opinion -- it comes across slightly dishonest if you insist that there is no limit even when it's obvious there must be one. (I mean seriously, how many machines is this running on right now? Even if it's running on amazon/google you can only scale my backend up so much with 20$/mo)