If I assign a single IAM role to multiple EC2 instances, do I get key rotations that are unique to each ec2 instance?
IAM role and Key Pairs are two different things.
IAM role (which gives you access to AWS services) will automatically expire and refresh credentials related to IAM. Nothing you need to do here.
Key Pairs which you use to log into the box are not rotated, and you would have to do that manually.