Depends on the number of classes you have already taken to get a 2.5 because if you have only taken 6 classes then get a 4.0 in you next 6 classes then your GPA with sky rocket but if you have taken...
I don't know how much you could raise it. I guess it would depend on how low it was and how much better your grades are now. Even if it doesn't improve a big amount, it is still worth it to try....