If every month, I am allowed to pay my rent anytime between the 20th of that month and the 30th of that month, then what to call that 10 days period?
I would use the term payment window.
Here is an example of its usage:
The Commons Treasury Committee has accused the chancellor of delaying introduction of the 30-day payment window for CGT on disposals of residential property to help the government meet its fiscal rule deadline in 2020/21.
The new requirement for payment on account within 30 days of completion of chargeable residential property disposals was originally announced at Autumn Statement 2015 and intended to take effect in April 2019. The Autumn Budget announced a deferral of the change by one year, until April 2020.