When should fstrim run?

Based on this cyanogenmod forum thread , I tried to address the delay in running applications and switching between applications on my Galaxy Nexus running cyanogenmod with Android 4.4.4 by running fstrim manually. root@maguro:/ # fstrim -v /data /data: 11649122304 bytes trimmed root@maguro:/ # fstrim -v /cache /cache: 436121600 bytes trimmed root@maguro:/ # fstrim -v /system /system: 80490496 bytes trimmed root@maguro:/ # exit This has largely fixed the delay issues. The delay to return to the home screen, for example, has reduced from about 4 seconds to less than 1 second. Based on this performance boost, I expect the delay was caused by the filesystem running inefficiently. Isn't the Android OS supposed to run fstrim on its own? When is this supposed to happen?