Tummy Tuck Revision for Muscles Still Lax

It sounds like the muscle layer is indeed lax after the repair.  To be sure, you may want to seek a second opinion, as it is not possible to make a diagnosis based on verbal description alone.
I would say that 9 months to a year after the initial surgery should allow adequate healing to consider a re-do.

From your comments I am guessing that you HAD a Tummy tuck and that you still have a residual pooch when you push out. That you operating surgeon suggest you camouflage with Spanx and that another plastic surgeon feels that the reason for the pooch was insufficient muscle tightening not fat ("little fat under skin that not an issue").
Since the second Plastic surgeon is the only one who examined you and he feels he could get you a better result, I cannot doubt him. I would advise you wait 6-8 months for the tissue to mature and achieve full healing and maintain stable wait. Revision tummy tuck is not as easy as the first time the operation is done because the plane of the surgery is scarred. Although an improvement is possible, I would make absolutely sure that the improvement is worth the risk.