If you have spinning drives, monitor the (raw) sector count stats. Add together reallocated sectors, uncorrectable and offline uncorrectable.

If you have good operations, replace when convenient after it hits 100. If you have poor operations (like in home use, with no backups and only ocassional SMART checks) replace if it hits 10 and try to run a full SMART surface scan before using a new drive.

For SSDs, good luck, I haven't seen prefailure indicators.