I canceled the subscription I am asking after how long time I can get the refund back.
Have you contacted Support yet? Refunds are not automatic. If you've contacted Support and they've told you that you're eligible for a refund, it usually takes 4-5 business days to be processed by your bank.
If you haven't yet done so, you'll need to contact Dropbox Suport directly. Visit the Support page while you're NOT signed in to a Dropbox account, including these forums, and you'll see an option for Billing issues. It's best to use an Incognito or Private browsing session to make sure you're not signed in.