It takes months for them to weaken & die, but in the end they will. They can not survive in freshwater, the green spotted definatly can't! The figure 8 needs only light brackish (around 1.004) so they'll live in freshwater for a couple of months, but will still die in the end.
If you want freshwater puffers, have a look around in this section, you'll find plenty. If you really want the figure 8 puffer, go brackish. It isn't that difficult to keep