Cron Job Expression to take snapshot in Each 15 Mins

Hello,

Could you please help me to create Cron Job expression to take snapshot in Each 15 mins in SLM Policy.

Currently I am using below format.
'curl -X PUT "localhost:9200/_slm/policy/policy-name?pretty" -H 'Content-Type: application/json' -d
'{
"schedule": "* /15 * * * ?",
"name": "<my-snap-{now/d}>",
"repository": "repo-name”,
"config": {"indices": ["
"]},
"retention": {"expire_after": "30d","min_count": 1,"max_count": 500}
}' '
Above configure seems not working. It is taking snapshot very frequently with this.

Could you also please provide suggestion on min count and max count of retention policy. I want to store data for 30 days and indices would be around 120 in this 30 days.