Acount expiration for unfilled account

Hello

I am living outside of the US and just activated my brand new ATT gophone SIM card. So I have an account and a wireless number but I can't log in to my account to top-up the balance because I can't receive the text message with the one time password.

Can anyone tell me how long accounts without any money on it last after activation? I won't be in the US before April 28.

Thanks in advance

Ulrich