There is no limit to the amount of time an EC2 instance can be stopped for.
The reason for the RDS stop limit is so that patches can be applied to the database engine and operating system. RDS is a managed service - AWS manage the server and so it's important to have it powered on in order to install patches.
I understand that you want to stop your EC2 to reduce costs. However, when you stop your EC2, you will not incur charges only for the EC2 instance but you will still be charged for the EBS storage associated with that instance. You can reduce your EBS charges while retaining the information stored in it for later use by creating a snapshot of it. Also refer to this document for other ways you can use to further reduce your cost for your unused EC2 instance.
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