The 25$ unlock after the person you've referred made their second payment. Then the credit will be applied to your next bill.
You can find the information about referral here
When your friend completes their second monthly payment, they will receive their referral bonus. From there, your own referral bonus will be applied on your next payment.
If you have multiple plans, your referral bonuses will be applied on the payment for your default main plan (the oldest plan in your account).You can change your settings under your profile to choose another plan. Once that change is done, any future referral bonus will be assigned to that other chosen plan.