My payment was canceled for icloud, how long would it take for my icloud plan to be downgraded?

Make sure you actually cancel the subscription. Don’t just not pay for it as that could have other potential bad outcomes.

Legal - iCloud - Apple

“The applicable fee for an upgraded plan will take effect immediately; downgrades to your plan will take effect on the next annual or monthly billing date. YOU ARE RESPONSIBLE FOR THE TIMELY PAYMENT OF ALL FEES AND FOR PROVIDING APPLE WITH VALID CREDIT CARD OR PAYMENT ACCOUNT DETAILS FOR PAYMENT OF ALL FEES. If Apple is unable to successfully charge your credit card or payment account for fees due, Apple reserves the right to revoke or restrict access to your stored content, delete your stored content, or terminate your Account. If you want to designate a different credit card or payment account or if there is a change in your credit card or payment account status, you must change your information online in the Account Information section of iCloud; this may temporarily disrupt your access to the Services while Apple verifies your new payment information. We may contact you via email regarding your account, for reasons including, without limitation, reaching or exceeding your storage limit of your selected plan.”

Help_qoejnh1 wrote:

I dont have enough money to pay monthly icloud right now, so i was wondering how long would it take before my 50GB gets canceled and i get the downgraded plan of 5GB?

According to the article that you were reading when you posted your question….

If you downgrade or cancel your iCloud+ plan, the change takes effect after your current subscription billing period ends.1