Using Lambda for the slaves is probably not the best idea because, as @saynay mentioned, you couldn't run builds any longer than a few minutes.

You could "wrap" a Jenkins fleet running on EC2 with Lambda functions to set up on a build request event and tear down on a build finish event, and theoretically this would reduce the amount of resources you consume if your builds are few and far between. Think of GitHub webhooks triggering Travis CI as an analogy.

Whether or not that would actually be useful is debatable.

It looks like Lambda functions are limited in complexity and runtime duration.

The way I see it is pairing it with the container service (ECS) and spawning containers in response to events, possibly spawning EC2 instances if you need more computer.

It looks like Lambdas can also be set to execute at a scheduled time, so you could make a delayed Lambda that would shut down any EC2 instances you spawned as well.