According to
https://www.openshift.com/products/pricing/plan-comparison the free plan idles after 24 hours of inactivity (
https://developers.openshift.com/en/managing-idling.html)
However you can work around it by hitting your server with an http request every 24 hours, or upgrade to the Bronze Plan which apparently has $0 base price anyway so I'm not sure what the downside is.