3 days is probably an overstatement, but I think keeping an EC2 instance around for a long time is a bad habit.
Most developers don’t like to think that their software leaks memory, resources, etc, but in fact it most likely does. Even if the software that you wrote does not leak, keep in mind the system your software runs on top of may leak.
I think there are about 200K lines of code (LOC) that make up Xen, the virtualization platform that Amazon uses, 500K LOC or so for the Java VM (if you are using Java) and 2.5M LOC for a typical distribution of Linux. That is a lot of software - surely there will be memory and resource leaks somewhere in there.
If there are leaks, the system will degrade over time. The longer you keep the system running, the slower it will get. EC2 makes this really easy to solve - just kill your instance every so often and launch a new fresh instance. This means you should be using custom AMIs, EBS and or S3 to store your data of course.
To ensure your application appears to the outside world as a stable system, you’ll need to use Elastic Load Balancing, or at least Elastic IP to create a stable fascade on top of your ever-changing EC2 instances.
