Well, if by "chassis", you mean the unibody, they will last indefinitely if you keep them clean and make sure you don't have serious rust starting. Of course, they can always be repaired, but that gets expensive.
Had to say without knowing where you reside......if you drive on highly salted roads, your floorpan life will me much shorter than my '92 out here in arid West Texas.. I've spent ample time under my Fox-bodies ('92 GT and the '86 Boss TBird) changing and adding suspension components and found very, very little rust...... You'll just have to examine your own ride for rust and corrosion. I would double-check the suspenion attachment points.