What is the TTL for A Records with an Application Load Balancer?

I'm a bit confused about the issue you're facing. Is it not possible to run your application on both EKS and ECS simultaneously? Given you're using EKS, I would assume your app can handle multiple replicas. You could even set up a target group in the old ALB with new members and balance weights. That way, the TTL for the DNS record might not even matter in that scenario.

You can check your Route 53 record by looking for the authoritative hosts. Running a command like 'dig @your-dns-server.amazon.whatever.route53.com your-alb.your-domain.com A' should help you find the information you need.

I've heard it's typically around 60 seconds for DNS caching, but I'm not entirely sure. It's always good to double-check as policies can vary.

I think the ALB for your EKS is managed by the AWS LB Controller, which provisions directly in your cluster. While reusing the old ALB is an option, moving to the new one might be the better long-term solution.

If your current DNS record is an ALIAS to the existing ALB, clients can cache it for up to 60 seconds when you change to the new ALB. But if it's a CNAME, then it will depend on the TTL of the existing CNAME. Most clients respect these TTLs, but some may have their own caching rules. To prevent issues, consider reusing the old ALB with different target groups and listener rules. This allows for a seamless transition without worrying about TTLs, and you can manage how connections drain.