[Question] Do Pre-Paid Sub Cards Stack?

(Mods, if this isn't allowed, please feel free to delete.)

Also, sorry if the following is a retarded question.

Hi, I was wondering if the $29.99 Pre Paid Sub Cards you can buy from places like 7-11 or Wal Mart can be stacked when you redeem em one after the other; so like if I were to go out & buy 4 of them; & I go to redeem em, would they stack up to 8 months?