It's the same code as the built in clean up process except with an additional filter on asset. Tested locally and seams to work so my guess is your asset isn't correct. The $asset variable is currently set to profile.%.field.138. This should be profile.%.field.FIELD_ID where FIELD_ID is the id of the field (found in CB > Field Management) of your activity field. If you changed that activity fields Asset parameter then the asset for activity stored in that field won't be profile.%.field.FIELD_ID. It will be whatever you set the Asset parameter to.

I was a bit lost with the $duration system.

You set $duration to how far back you want to allow entries to remain. It's currently set to -3 DAYS in my example. This means any post older than 3 days will be deleted. If you need 5 days for example you'd set it to -5 DAYS. This is using PHP DateTime object API. See the below documentation on how to use relative formats as there's a lot of flexibility in it.

www.php.net/manual/en/datetime.formats.relative.php