just called and got my $20 a month for 6 month credit..... you may be able to get bigger ones depending on how many credits you currently have on your account, i already have $50+ dollars in credits so that standard $20 a month for 6 months was all I could get out them.
TIP: Direct TV employees get their credit banks filled on Mondays, when they run out of credits for the week there is no credits left. Good Luck!