As of October 2, 2018 you will no longer be able to add credits to your Envato Market. You can read more here.
Credits or "Envato credits" active on your Envato Market account are valid for 12 months.
If you do not use your Envato credits they will expire and be removed from your account.
Four weeks before your credits are due to expire, you will receive a reminder email. If you log into your account during this period, you will see a banner displayed at the top of the screen that will also give you the option to extend your credits (if they have not yet expired).