Hi Jenish,

Adding to  Nam Nguyen Phuong's Answer,

Leave allocations without an end date will stay available for the employee to use until they are consumed. 

For instance, if an employee is given 10 days of paid leave without an end date, those 10 days will be available for use indefinitely until the employee takes leave for a total of 10 days. 

if there is an end date specified for the leave allocation, the allocated leave days will no longer be available for use after that particular date, and the employee must utilize them before the specified end date.

This is from Odoo v15 Onwards, Earlier end date is added in the leave type

Hope it helps,
Kiran K