You can now set a default instance warm-up time for all scaling activities, health check replacements, and other replacement events in the Auto Scaling instance lifecycle. Amazon EC2 Auto Scaling is a service that allows you to automatically scale and manage logical groups of instances, known as Auto Scaling groups, that serve your application. EC2 Auto Scaling does this by monitoring various metrics, such as CPU utilization and application demand, to determine if an instance needs to be replaced, removed from, or added to your Auto Scaling group. Setting the default instance warm-up time parameter can simplify your Auto Scaling group configuration by ensuring that any scaling and replacement policies are aware of the time your instances typically take to be ready to serve demand.
Source:: Amazon AWS