1. A method, comprising: identifying, in a storage system comprising multiple storage devices having respective sets of storage regions, respective default low storage region thresholds that are used for garbage collection; defining, for each given storage device, a time threshold and an alternative low storage region threshold greater than the default low storage region threshold for the given storage device; and while processing input/output operations for each given storage device: maintaining a count of unused storage regions in the given storage device, initializing a timer, and initiating a garbage collection operation upon the timer matching the time threshold for the given storage device.