Though it's not named "13 Rounds"; the few reports on the sequel possibilities were all published prior to the film's release. Moviehole reports that it depends on the financial performance of the flick. Given that at least half of the box office revenue goes to the cinema owners (it's budget is around $20m) and with the DVD reception not being great either, the onlt thing that's clear is that WWE Studios would be lucky if they break even with 12 Rounds, meaning it's unlikely that the sequel would be made.
While it looks like we won't be seeing a "13 Rounds" any time soon, Cena has plenty of film-work up his sleeve. Besides starring in an emotional WWE Studios-produced film called Brother's Keeper, Cena is also trying to land a role in the cast of a remake of the 1987 Schwarzengger flick Predator. Keeping my fingers crossed.