CF will still resolve it to prevent website downtime when migrating away from CF. I can’t say the exact time of when this will be completely resolved, but I would bet 7 days.
I would highly recommend changing the nameservers. There are bots that look for unused domains pointed to Cloudflare and then use multiple CF accounts to try to take these domains over and use them to backlink spam or serve adware. The two nameservers combination you got is used among tens of thousands of customers, so any one could just add the domain to Cloudflare and they would instantly gain control of it since the nameservers already point to the ones CF asks them to point it to.
The only way to prevent this is having it in your CF account (it will generate a different combination for these bots if it’s in your account), changing the nameservers to a different system, or not having any nameservers.