Do Amazon Docs:
By setting a concurrency limit on a function, Lambda guarantees that allocation will be applied specifically to that function, regardless of the amount of traffic processing remaining functions. If that limit is exceeded, the function will be throttled. How that function behaves when throttled will depend on the event source. For more information, see Throttling Behavior
Então, a partir de documentos aws lidando com o comportamento de limitação: link
On reaching the concurrency limit associated with a function, any further invocation requests to that function are throttled, i.e. the invocation doesn't execute your function. Each throttled invocation increases the Amazon CloudWatch Throttles metric for the function. AWS Lambda handles throttled invocation requests differently, depending on their source:
Synchronous invocation: If the function is invoked synchronously and is throttled, Lambda returns a 429 error and the invoking service is responsible for retries. The ThrottledReason error code explains whether you ran into a function level throttle (if specified) or an account level throttle (see note below). Each service may have its own retry policy. For example, CloudWatch Logs retries the failed batch up to five times with delays between retries. For a list of event sources and their invocation type, see Supported Event Sources.
Asynchronous invocation: If your Lambda function is invoked asynchronously and is throttled, AWS Lambda automatically retries the throttled event for up to six hours, with delays between retries. Remember, asynchronous events are queued before they are used to invoke the Lambda function.
Portanto, se você definir um limite concorrente (o padrão é 1000 em todas as suas funções), a AWS fornecerá um código de status 429 (para solicitação-resposta) ou automaticamente e tentará novamente sua função para até 6 horas.
Ele não especifica como funciona a função de atraso entre novas tentativas.