Chances of Septum Deviation Due to Memory?

It depends on the technique used in the septoplasty. Most commonly a septal resection is used which removes the deviated portion of the cartilage. This way the deviated septum should not reccur. Scorring of the cartilage has less effect and might reccur but it is also used in milder cases.

I also had a 3 hour open septoplasty (no rhinoplasty) 8 weeks ago today. The resulthas been great. The chance of your cartilage warping and re-obstructing would depend on what was done. In my case most of the cartilage was removed leaving only an "L strut" to support the dorsum and columella. A columellar strut graft was also placed for further support. I also had spreader grafts and the small remaining piece of cartilage was replaced between the leaves of septal mucosa to minimize baffling on inspiration.
Sometimes, the septum is retained and repositioned and I would think this might have a higher chance of warping but you should ask your surgeon for the details of your surgery. Good luck and happy breathing!