The earlier you make an Employment Allowance Claim, the sooner you get it.
In case you make a late claim and have a huge unused Allowance, you'll need to ask HMRC to do one of the following:

• offset the unused balance against other current or future PAYE liabilities so, wherever possible, the unused balance of the allowance does not go waste.
• give you a refund of any unused balance of the allowance provided you do not have any other liabilities.

For more detailed information, refer to the HMRC link Claiming Employment Allowance: further employer guidance