A burstable performance instance configured as unlimited, such as a T3 instance, can sustain high CPU utilization for any period of time whenever required. The hourly instance price can automatically cover all CPU usage spikes. It does so if the average CPU utilization of the instance is at or below the baseline over a rolling 24-hour period or the instance lifetime, whichever is shorter.
For the vast majority of general-purpose workloads, instances configured as unlimited give enough performance without any additional charges. If the instance runs at higher CPU utilization for a prolonged period, it can do so for a flat additional rate per vCPU-hour. For information about T3 instance pricing, see "T3 CPU Credits" in AWS EC2 Service.
For more information on unlimited mode for T3 instances, see Unlimited mode for burstable performance instances in the Amazon EC2 User Guide for Linux Instances.