If Juan Gonzalez can keep getting people to sign him to contracts despite the fact that he's been constantly hurt every single year since like 2001 or so, then Frank can keep getting people to sign him to DH for at least 5 years.
Even if he only ends up playing half a season for whatever team he signs for...he's still probably going to end up with 20 home runs in that half of a season, and he'll give that lineup a major punch.
Even if he's getting hurt a fair amount, he's still going to wind up being a dominant hitter. And if he keeps his weight under better control like he is this year (fish diet?), then there's no reason he can't stay around for a long time.