The part of the SMCC molecule that us unstable is the NHS ester. Once that has reacted with lysines in your antibody, the maleimide group in the maleimide-derivatized antibody is stable, assuming you desalt into a buffer with the correct pH.

If you desalt into an acidic buffer at around pH 6 and then freeze small aliquots then, yes, the maleimides will last for many weeks, even months.

However, you have the antibody to think about here too. Is that going to be stable under these conditions? pH 6 will normally be okay; snap freezing, as opposed to slow freezing, is often required for proteins, so freeze on nitrogen or dry ice/ethanol. However, direct transfer to -80 may be okay for small aliquots.

For the maleimide, however, the main thing is to avoid any conditions above pH 8. Note: NHS ester reactions are often done at pH 8, but they do not have to be. A slower reaction at pH 7.2-7.4 is perfectly fine.