You don't have to enable anything. The fstrim.timer his enabled by default and runs fstrim.service once per week.

That command will issue fstrim on all mounted file systems listed in fstab. It has no effect on HDD, or conventional "thick" lvm. But it does have an effect on thin volumes, it will return no longer used blocks back to the thin pool.

Once per week is enough for most workloads.