Systems and methods are described for adjusting a number of concurrent code executions allowed to be performed for a given user on an on-demand code execution environment or other distributed code execution environments. Such environments utilize pre-initialized virtual machine instances to enable execution of user-specified code in a rapid manner, without delays typically caused by initialization of the virtual machine instances. However, to improve utilization of computing resources, such environments may temporarily restrict the number of concurrent code executions performed on behalf of the given user to a number less than the maximum number of concurrent code executions allowed for the given user. Such environments may adjust the temporary restriction on the number of concurrent code executions based on the number of incoming code execution requests associated with the given user.